Mechanisms of chromosome banding. VII. Interaction of methylene blue with DNA and chromatin

Bisbenzimide compounds inhibit the replication of prototype and pandemic potential poxviruses

We previously identified the bisbenzimide Hoechst 33342 (H42) as a potent multi-stage inhibitor of the prototypic poxvirus, the vaccinia virus (VACV), and several parapoxviruses. A recent report showed that novel bisbenzimide compounds similar in structure to H42 could prevent human cytomegalovirus replication. Here, we assessed whether these compounds could also serve as poxvirus inhibitors. Using virological assays, we show that these bisbenzimide compounds inhibit VACV spread, plaque formation, and the production of infectious progeny VACV with relatively low cell toxicity. Further analysis of the VACV lifecycle indicated that the effective bisbenzimide compounds had little impact on VACV early gene expression but inhibited VACV late gene expression and truncated the formation of VACV replication sites. Additionally, we found that bisbenzimide compounds, including H42, can inhibit both monkeypox and a VACV mutant resistant to the widely used anti-poxvirus drug TPOXX (Tecovirimat). Therefore, the tested bisbenzimide compounds were inhibitors of both prototypic and pandemic potential poxviruses and could be developed for use in situations where anti-poxvirus drug resistance may occur. Additionally, these data suggest that bisbenzimide compounds may serve as broad-activity antiviral compounds, targeting diverse DNA viruses such as poxviruses and betaherpesviruses.IMPORTANCEThe 2022 mpox (monkeypox) outbreak served as a stark reminder that due to the cessation of smallpox vaccination over 40 years ago, most of the human population remains susceptible to poxvirus infection. With only two antivirals approved for the treatment of smallpox infection in humans, the need for additional anti-poxvirus compounds is evident. Having shown that the bisbenzimide H33342 is a potent inhibitor of poxvirus gene expression and DNA replication, here we extend these findings to include a set of novel bisbenzimide compounds that show anti-viral activity against mpox and a drug-resistant prototype poxvirus mutant. These results suggest that further development of bisbenzimides for the treatment of pandemic potential poxviruses is warranted.

Insight into magnesium ions effect on chromosome banding and ultrastructure

Magnesium ion (Mg²⁺ ) plays a fundamental role in chromosome condensation which is important for genetic material segregation. Studies about the effects of Mg²⁺ on the overall chromosome structure have been reported. Nevertheless, its effects on the distribution of heterochromatin and euchromatin region have yet to be investigated. The aim of this study was to evaluate the effects of Mg²⁺ on the banding pattern and ultrastructure of the chromosome. Chromosome analysis was performed using the synchronized HeLa cells. The effect of Mg²⁺ was evaluated by subjecting the chromosomes to three different solutions, namely XBE5 (containing 5 mM Mg²⁺ ) as a control, XBE (0 mM Mg²⁺ ), and 1 mM EDTA as cations-chelator. Chromosome banding was carried out using the GTL-banding technique. The ultrastructure of the chromosomes treated with and without Mg²⁺ was further obtained using SEM. The results showed a condensed chromosome structure with a clear banding pattern when the chromosomes were treated with a buffer containing 5 mM Mg²⁺ . In contrast, chromosomes treated with a buffer containing no Mg²⁺ and those treated with a cations-chelator showed an expanded and fibrous structure with the lower intensity of the banding pattern. Elongation of the chromosome caused by decondensation resulted in the band splitting. The different ultrastructure of the chromosomes treated with and without Mg²⁺ was obvious under SEM. The results of this study further emphasized the role of Mg²⁺ on chromosome structure and gave insights into Mg²⁺ effects on the banding distribution and ultrastructure of the chromosome.

Inhibition of Poxvirus Gene Expression and Genome Replication by Bisbenzimide Derivatives

Virus infection of humans and livestock can be devastating for individuals and populations, sometimes resulting in large economic and societal impact. Prevention of virus disease by vaccination or antiviral agents is difficult to achieve. A notable exception was the eradication of human smallpox by vaccination over 30 years ago. Today, humans and animals remain susceptible to poxvirus infections, including zoonotic poxvirus transmission. Here we identified a small molecule, bisbenzimide (bisbenzimidazole), and its derivatives as potent agents against prototypic poxvirus infection in cell culture. We show that bisbenzimide derivatives, which preferentially bind the minor groove of double-stranded DNA, inhibit vaccinia virus infection by blocking viral DNA replication and abrogating postreplicative intermediate and late gene transcription. The bisbenzimide derivatives are potent against vaccinia virus and other poxviruses but ineffective against a range of other DNA and RNA viruses. The bisbenzimide derivatives are the first inhibitors of their class, which appear to directly target the viral genome without affecting cell viability.

IMPORTANCE Smallpox was one of the most devastating diseases in human history until it was eradicated by a worldwide vaccination campaign. Due to discontinuation of routine vaccination more than 30 years ago, the majority of today's human population remains susceptible to infection with poxviruses. Here we present a family of bisbenzimide (bisbenzimidazole) derivatives, known as Hoechst nuclear stains, with high potency against poxvirus infection. Results from a variety of assays used to dissect the poxvirus life cycle demonstrate that bisbenzimides inhibit viral gene expression and genome replication. These findings can lead to the development of novel antiviral drugs that target viral genomes and block viral replication.

Differential staining of dysplastic aberrant crypt foci in the colon facilitates prediction of carcinogenic potentials of chemicals in rats

We developed a novel and simple method to identify dysplastic aberrant crypt foci (ACF) induced in rats by colon carcinogens more efficiently and selectively without conducting laborious histological examination, which usually requires enough time to get final diagnosis. By adding a simple decolorization process with 70% methanol after conventional 0.2% methylene blue staining, dysplastic ACF could be differentially contrasted. To examine the validity of this novel method, which we refer to as differential staining, we analyzed colonic lesions induced by three heterocyclic amines, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, and found that the number of dysplastic ACF detected more precisely reflected their carcinogenic potential than the total numbers of ACF.

The structure of human metaphase chromosomes: its histological perspective and new horizons by atomic force microscopy

Studies on the structure of the human chromosome were reviewed from the histological perspective and discussed in connection with our recent findings obtained mainly by atomic force microscopy (AFM). In this paper, we introduce several hitherto known models of the high-order structure of the metaphase chromosome and discuss the actual structure of chromosomes in relation to such structures as spiral chromatids, chromosome bands, and chromosome scaffolds. In chromosomes treated with Ohnuki's hypotonic solution, the chromosome arms were elongated and showed a characteristic spiral pattern of chromatid fibers. On the other hand, alternating transverse ridges and grooves were clearly observed on the surface of chromosomes treated with 0.025% trypsin for G-banding, and these ridges and grooves corresponded to the dark and pale bands of G-banded chromosomes. Similar findings were also found in chromosomes treated with quinacrine mastards for Q-banding. Fibers bridging the gap between the sister chromatids were often observed in G/Q-banded chromosomes; these fibers tended to be restricted within the G/Q-positive portions, suggesting the presence of chromatin fibers bridging these regions. Based on these findings in conjunction with previous studies, we outlined the high-order structure of the human chromosome. Recent advances in nanotechnology have provided new AFM techniques for the imaging and handling of materials at nano-scale resolution. Application of these techniques to chromosome research is expected to provide valuable information on the chromosome structure in relation to its function.

Three-dimensional helical coiling structures and band patterns of hydrous metaphase chromosomes observed by low vacuum scanning electron microscopy

Helical coiling structures and band patterns of hydrous metaphase chromosomes were documented three-dimensionally by low vacuum scanning electron microscopy (SEM). Fixed or unfixed isolated Chinese hamster metaphase chromosomes were stained with platinum blue (Pt blue) and observed in the backscattered electron mode for low vacuum SEM without any hypotonic treatment or drying processes. Fibrous structures were shown both in the fixed and unfixed hydrous chromosomes; helical chromatid coils and their subcoils were clarified especially in the fixed chromosomes having contrasting alternative bands of light and darkness, while the translucent perichromosomal matrix and compact fibrous structures were recognized in the unfixed chromosomes. The helical coils were more clearly represented in a loosened chromatid of metaphase chromosomes. Treatment with a tris-HCl buffer solution and Pt blue staining in a hydrous condition successfully produced banding patterns similar to G-bands on metaphase chromosomes. These banded chromosomes observed by low vacuum SEM were also analyzed stereoscopically by field emission SEM after critical point drying. These findings indicate that: 1) native or unfixed chromosomes maintain the compact arrangement of high-order helical structures covered with the peri-chromosomal matrix; 2) helical coiling appearances of chromatids frequently observed in previous papers might be caused by loosening of the final level of the high-order structure of the metaphase chromosome; and 3) banding patterns might be produced by the rearrangement or reorganization of chromatin fibers at the 30 nm fiber level after the extraction of some chromosomal components including the peri- or intra-chromosomal materials during the banding procedure.

Evolution of chromosome bands: molecular ecology of noncoding DNA

Giemsa dark bands, G-bands, are a derived chromatin character that evolved along the chromosomes of early chordates. They are facultative heterochromatin reflecting acquisition of a late replication mechanism to repress tissue-specific genes. Subsequently, R-bands, the primitive chromatin state, became directionally GC rich as evidenced by Q-banding of mammalian and avian chromosomes. Contrary to predictions from the neutral mutation theory, noncoding DNA is positionally constrained along the banding pattern with short interspersed repeats in R-bands and long interspersed repeats in G-bands. Chromosomes seem dynamically stable: the banding pattern and gene arrangement along several human and murine autosomes has remained constant for 100 million years, whereas much of the noncoding DNA, especially retroposons, has changed. Several coding sequence attributes and probably mutation rates are determined more by where a gene lives than by what it does. R-band exons in homeotherms but not G-band exons have directionally acquired GC-rich wobble bases and the corresponding codon usage: CpG islands in mammals are specific to R-band exons, exons not facultatively heterochromatinized, and are independent of the tissue expression pattern of the gene. The dynamic organization of noncoding DNA suggests a feedback loop that could influence codon usage and stabilize the chromosome's chromatin pattern: DNA sequences determine affinities of----proteins that together form----a chromatin that modulates----rate constants for DNA modification that determine----DNA sequences. Theories of hierarchical selection and molecular ecology show how selection can act on Darwinian units of noncoding DNA at the genome level thus creating positionally constrained DNA and contributing minimal genetic load at the individual level.

The involvement of nucleosomes in Giemsa staining of chromosomes. A new hypothesis on the banding mechanism

A new hypothesis is proposed on the involvement of nucleosomes in Giemsa banding of chromosomes. Giemsa staining as well as the concomitant swelling can be explained as an insertion of the triple charged hydrophobic dye complex between the negatively-charged super-coiled helical DNA and the denatured histone cores of the nucleosomes still present in the fixed chromosomes. New cytochemical data and recent results from biochemical literature on nucleosomes are presented in support of this hypothesis. Chromosomes are stained by the Giemsa procedure in a purple (magenta) colour. Giemsa staining of DNA and histone (isolated or in a simple mixture) in model experiments results in different colours, indicating that a higher order configuration of these chromosomal components lies at the basis of the Giemsa method. Cytophotometry of Giemsa dye absorbance of chromosomes shows that the banding in the case of saline pretreatment is due to a relative absence of the complex in the faintly coloured bands (interbands). Pretreatment with trypsin results in an increase in Giemsa dye uptake in the stained bands. Cytophotometric measurements of free phosphate groups before and after pretreatment with saline, reveal a blocking of about half of the free phosphate groups indicating that a substantial number of free amino groups is still present in the fixed chromosomes. Glutaraldehyde treatment inhibited Giemsa-banding irreversibly while the formaldehyde-induced disappearance of the bands could be restored by a washing procedure. These results correlate with those of biochemical nucleosome studies using the same aldehydes. Based on these findings and on the known properties of nucleosomes, a mechanism is proposed that explains the collapse of the chromosome structure when fixed chromosomes are transferred to aqueous buffer solutions. During homogeneous Giemsa staining reswelling of the unpretreated chromosome is explained by insertion of the hydrophobic Giemsa complex between the hydrophobic nucleosome cores and the superhelix DNA. Selective Giemsa staining of the AT-enriched bands after saline pretreatment is thought to be due to the, biochemically well-documented, higher affinity of arginine-rich proteins present in the core histones for GC-enriched DNA, which prevents the insertion of the Giemsa complex in the interbands. Production of Giemsa bands by trypsin pretreatment can be related to the action of this enzyme on the H1 histones and subsequent charge rearrangements.(ABSTRACT TRUNCATED AT 400 WORDS)

Methylene blue: effects and disposition in sheep

The disposition and urinary excretion pharmacokinetics of methylene blue were determined after its intravenous administration at 15 mg/kg to mature female sheep. Comparisons were made between methylene blue administered alone or subsequent to 50 mg/kg sodium nitrite. The overall elimination rate constant (beta) of methylene blue, 0.0076 +/- 0.0016 min-1, was not influenced by prior administration of sodium nitrite. However, the distribution rate was significantly altered by sodium nitrite. Very little of the methylene blue was eliminated in the urine either intact or as leucomethylene blue in spite of its relatively short half life. Toxicologic assessment was carried out using LD50 determination, methemoglobin production and hematologic effects as evaluation parameters. Methemoglobin production was minimal with doses as high as 50 mg/kg and no significant hematologic changes were seen up to 4 weeks after a total dose of 30 mg/kg methylene blue. The 24 h LD50 for intravenous methylene blue administered as a 3% solution was 42.3 mg/kg with 95% confidence interval limits of 37.3 to 47.9 mg/kg. From these data it appears that as conditions may warrant, the dosage of methylene blue may be safely increased up to at least 15 mg/kg in therapy of severe methemoglobinemias.

G-bands without pretreatment of slides, in chemically defined conditions

We investigated the capability of Giemsa dyes to produce G-bands without pretreatment of slides in chemically defined conditions. G-banding was produced between pH 7.0 and pH 11.0. Within this range, we observed that the achievement of G-bands was dependent on the ionic strength and time of staining. From these data and from those of other authors on other chemical variables and on the mechanisms of staining, we propose a working hypothesis of G-band formation, with a step involving shrinkage of the chromatids and rearrangement of the chromosome fiber during the staining process.

Proteins in chromosome banding. II. Effect of R- and C-banding treatments on the proteins of isolated nuclei

SDS polyacrylamide gel electrophoresis was used to study the proteins extracted from, and those remaining in isolated, fixed, air-dried nuclei subjected to a variety of R- and C-banding techniques. The R-banding procedures, involving exposure to hot Earle's BSS or NaH2PO4, had the least effect of any of the banding techniques on the extraction of proteins from isolated nuclei. Only small amounts of 5 nonhistone proteins were detected in the Earle's BSS extract, and no proteins were found in the NaH2PO4 solution after treatment. The residual proteins remaining in the nuclei after either treatment were virtually identical to those in control nuclei. The C-banding techniques, on the other hand, produced substantial changes in the nuclear proteins. These techniques involve several sequential steps, including HCl treatment, exposure to NaOH or Ba(OH)2, and an incubation in hot SSC. The HCl treatment extracted a large variety of nonhistones and some of each of the remaining histones. No proteins were detected in the SSC solution. Some of the proteins extracted by Ba(OH)2 appeared after the two complete C-banding treatments revealed both similarities and differences. The Ba(OH)2 technique appeared to have a more severe effect on the nuclear proteins than the NaOH technique. Fewer residual nuclear proteins were observed after the former technique, but all of these were also represented in nuclei after the NaOH technique. The results indicate that the different treatments producing a common type of banding generally have similar effects on the nuclear proteins, while the treatments producing different types of banding (G-, R-, C-banding) have substantially different effects on these proteins. Such alterations may have implications for chromosome banding.

Proteins in chromosome banding. I. Effect of G-banding treatments on the proteins of isolated nuclei

Nuclei were isolated from Chinese hamster cells, treated with hypotonic KCl, fixed in acetic methanol, and either air-dried in glass tubes (in situ) or left in suspension (in vitro). These preparations were then exposed to a variety of G-banding treatments, including the 2 xSSC, urea, NaCl-urea, and trypsin methods. The proteins extracted into the treatment solution and those remaining in the nuclei were analyzed by SDS polyacrylamide gel electrophoresis. The three former treatments extracted specific subsets of the total nuclear nonhistone proteins into the treatment solution. Some of the extracted nonhistones were common to all treatments while others were unique to a particular treatment. Variable amounts and types of the histones were also extracted by these treatments, but significant quantities of all of these proteins still remained in the nuclei afterwards. The trypsin treatment appeared to degrade some of the nonhistones, while other nonhistones, as well as the histones, were relatively resistant to trypsin digestion. Although there were a few differences in the residual proteins found in the nuclei after the various G-band treatments, the overall electrophoretic patterns of these proteins were generally similar. The results indicate that the G-banding techniques induce specific and reproducible changes in the proteins of isolated nuclei. If these banding treatments induce similar changes in the proteins of mitotic chromosomes, such alterations might be involved in mechanisms of chromosome banding.

Effects of acetic acid-alcohol, trypsin, histone 1 and histone fragments on Giemsa staining patterns in chromosomes

In an effort to minimize subjective bias, a classification scheme was devised to assess Giemsa staining patterns obtained with experiments involving acetic acid-alcohol and exogenously applied histone 1 and polypeptides. A single rinse of metaphase preparations with acetic acid-alcohol quantitatively reduced Giemsa dye binding. Acid-alcohol irreversibly changed the conformation of H1 and its ability to interfere with trypsin G-banding. Our results suggest that, in addition to protein extraction, acid-alcohol may alter the conformation of acid-insoluble components of metaphase chromosomes. The carboxy-terminal polypeptide (residues 73--212) from NBS cleavage of H1 was an effective inhibitor of Giemsa staining and trypsin G-banding. However, this polypeptide which is preferential for supercoiled DNA was much less efficient in inhibiting Giemsa staining of trypsinized metaphase chromosomes. The molecular consequences of these experiments are discussed.

Tissue-specific changes in nuclear RNA content during early development of Triturus vulgaris

During early development of Triturus vulgaris, as a measure for nuclear activity in neuroectoderm, mesoderm and endoderm, nuclear RNA content was determined by cytochemical methods. In the first stages of gastrulation, that is to say during the early phase of neural induction, the RNA content of the inducing system is considerably higher than in the reacting system. Then, with a phase-shift of about 10 h, the RNA content of the neuroectoderm increases quickly also. In the following stages the nuclear RNA content of both regions is reduced. A second continuous increase in the RNA amount coincides with the formation of the neural tube. In the mesoderm, enhancement of RNA content correlates with cytodifferentiation of the chorda. In all stages the RNA content of the endoderm is higher than in the other tissues and it becomes successively diminished from the early gastrula to the tailbud stage.

The mechanism of C-binding: depurination and beta-elimination

C-banding of chromosomes involves the differential solubilization of fragmented DNA from euchromatin by three sequential treatments: 1. Acid, 2. Mild base, 3. Hot salt. The data indicate solubilization is effected by 1) depurination, 2) DNA denaturation, 3) chain breakage of the depurinated sites respectively in the three treatments. Conditions were found wherein each treatment in proper sequence was necessary for C-banding and the appropriate chemical reactions were measured in these treatment conditions. The acid treatment (0.2 N HCl) depurinates chromosomal DNA at the rate of 0.26 x 10(-6) purines/dalton min to an alkaline molecular weight of 10(5) daltons but does not break the depurinated sites. Bleomycin can substitute for acid as a base removing agent. Sodium borohydride, by reducing the depurinated sugar's aldehyde thereby preventing chain breakage by the beta-elimination reaction, reversibly inhibits DNA-extraction. Chain breakage at the DNA's apurinic sites occurs not in the 2 min mild alkali treatment where the half-life for breakage is 26 min but in the 18 h hot salt treatment where the half-life for chain breakage is 1-2 h. Most of the DNA extraction occurs in the hot salt as 10(5) dalton fragments as measured in formamide gradients. Bleomycin is introduced as a substitute for HCl; it removes nitrogenous bases from DNA in situ while better preserving the morphology of the final C-banded chromosomes.

A modified Giemsa C-banding technique for Hordeum species

A Giemsa C-banding technique with a hot 1 N HCl hydrolysis step has been developed for barley chromosomes. This step makes it easy to obtain well separated C-banded chromosomes. To compare this technique with other C-banding techniques, chromosomes of H. vulgare cv. York were stained by both this technique and a modification of the technique of Kimber et al. (1976). With respect to centromeric and intercalary bands, both techniques produce a similar banding pattern, but telomeric bands observed by the modified technique of Kimber et al. (1976) were not detected by our procedure. This indicates that telomeric heterochromatin may be different chemically and/or structurally from the centromeric and intercalary heterochromatin and its appearance dependent upon the C-banding technique. The procedure described provides a relatively rapid technique for C-banding of barley chromosomes.

C-BANDING OF RYE CHROMOSOMES WITH COLD SSC BUFFER

Various times, temperatures and concentrations of SSC were tested in an attempt to elucidate the mechanism of C-banding in plants. It is shown that C-bands can be induced in rye (Secale cereale L.) chromosomes by SSC treatment at temperatures as low as 0 °C for periods as short as 1 min, an effect previously unknown in either plants or animals. Barium hydroxide treatment appears to be essential for the production of bands. If chromosomes are treated with SSC omitting the Ba(OH)2 treatment, relatively uniform loss of nucleoproteins occurs without the production of C-bands. It is suggested that Ba(OH)2 alters the chemical structure of nucleoproteins in heterochromatin rendering them insoluble in SSC. It is unlikely that SSC functions as a DNA reassociation agent in the production of C-bands. More likely it functions as a leaching agent which extracts soluble nucleoproteins from the chromosomes. Incubation in 2 × SSC at room temperatures for 5-10 min was found to be sufficient for the production of a well contrasted banding pattern in rye chromosomes.

Histones and G banding of chromosomes

Polylysine, polyarginine, and histones H1, H2A, H2B, and H3 inhibit Giemsa staining and chromosome banding by binding to DNA and preventing side stacking of the positively charged thiazine dyes to the negatively charged phosphate groups on DNA. This is a nonspecific effect and does not of itself provide evidence for a role of histones in G banding. The question of whether histones are involved in chromosome banding is reviewed.

Configurational changes in chromatids from helical to banded structures

Induction of configurational changes in the helical chromatids of air dried chromosomes was used to explore the mechanism of G-banding. From the water-Giemsa stained metaphase spreads of Chinese hamster cells, chromosomes having clearly helical chromatids were selected and photographed. Then the chromosomes were decolorized, treated with trypsin, and restained with saline-Giemsa (1X SSC). Such procedures were repeatedly carried out upon the same chromosomes. Subsequent examination of the chromosomes showed that configurational changes from a helical structure to a banded structure had occurred. Some chromosomes revealed a variety of transitional changes between these two configurations. During the repeated G-banding treatments, the distances between bands along the same chromatids changed each time. The results obtained seem to indicate that the G-banding results from locally induced compaction of chromosomal materials along the chromatids.

Banding of human chromosomes with basic fuchsin

In this paper a technique is described for the banding of human metaphase chromosomes with basic fuchsin. The main characteristics of the G-banding pattern obtained with this cationic triphenylmethane dye are: the secondary constriction regions of chromosomes Nos. 1 and 16 are strongly stained, especially in the latter one; the heterochromatic area of chromosome No. 9, usually negative with most other G-banding techniques, is clearly visible as an intensely stained band adjacent to the centromere; the chromosomal outline is often very distinct, facilitating the study of the telomeres; a number of chromosomal regions with bright Q fluorescence such as the polymorphic regions of the chromosomes Nos. 3, 4, and Y also stain strongly with basic fuchsin. The basic fuchsin technique combines therefore properties of G-, C-, and Q-banding methods and seems very suitable for use in e.g., family and linkage studies. Several triphenylmethanes closely related to basic fuchsin produce similar banding patterns. The band-producing ability is, however, diminished in those dyes which contain methylated amino groups. If the methyl groups are attached to the carbon atoms at the 3-positions in the phenyl rings, band formation seems unaffected. The way in which basic fuchsin and chromatin may interact as well as the possible mechanism(s) of band formation with this dye are discussed.

CT banding of human chromosomes: description of the banding technique and some of its modifications

A technique is described for staining centromeric areas and reverse, mainly telomeric bands in human chromosomes. With this "CT" technique karyotyping of C-banded metaphases is possible without previous or subsequent use of other banding methods. The method consists of an alkaline pretreatment at 60 degrees C with Ba(OH)2, followed by salt incubation in 2 X SSC at 60 degrees C and staining with the cationic dye "Stains-all". In a series of experiments the influence of the variables in the procedure was studied, with the following main results: 1) Ba(OH)2 treatment alone and subsequent staining produces a distinct reverse banding pattern in which the secondary constriction of chromosome 9 is positive. 2) The 2 X SSC incubation in the CT procedure causes the Ba(OH)2 induced reverse bands to become weaker; the centromeric regions, however, become very prominent. 3) If the temperature of the 2 X SSC treatment is raised to 85 degrees C, the CT technique results in a specific staining of the short arm regions of some probably variant acrocentric chromosomes. The interphase nuclei of individuals possessing such acrocentrics usually show very distinct chromocentres after this treatment; in the polymorphs these chromocentres are often situated along the nuclear membrane. The mechanisms which may form the basis of the staining results obtained, and the possible significance in human cytogenetics of the techniques described, are discussed briefly.