[Preliminary outcome for patients with relapsed or resistant advanced non-Hodgkin's lymphoma treated by EPOCH regimen]

BACKGROUND & OBJECTIVE

In the management of relapsed or refractory non-Hodgkin's lymphoma(NHL), there is still no standard salvage chemotherapy regimen so far. Potentiation of effectiveness and reduction of toxicity for some anti-cancer agents through continuous intravenous infusion were shown in some pre-clinical and clinical studies. The purpose of this study was to evaluate the efficacy and toxicity of EPOCH regimen.

METHODS

A prospective phase II study of EPOCH regimen (doxorubicin/epirubicin,vincristine,etoposide over 96 hours infusion with bolus cyclophosphamade and oral prednisone) was administered to 26 patients with relapsed or refractory/resistant aggressive NHL. There were 20 patients (84.7%) among them treated by over 2 kinds of chemotherapy regimen with median regimen types of 2 (range,1-6 types) and median chemotherapy cycles of 8 (range,3-16 cycles) given for all patients. Fifteen patients (65.7%) were chemo-resistant recurrence.

RESULTS

All the 26 patients were assessable. The response rate for the whole group was 50.0% with complete remission (CR) rate of 19.2%. Among the 7 patients with T-cell lymphoma and the 19 patients with B-cell lymphoma, the response rates were 28.6% and 57.9%,respectively. Major toxicity was myelosuppression with 34.8% and 8.7%incidence of grade III-IV neutropenia and thrombocytopenia respectively in all 46 cycles of EPOCH. Other toxicities were mild.

CONCLUSION

EPOCH was effective for the patients with relapsed or refractory/resistant aggressive non-Hodgkin's lymphoma. Continuous infusion schedules of several chemotherapeutic agents may partially reverse chemo-resistance and reduce toxicity. EPOCH can be used as standard salvage regimen. Further clinical study is warranted.

[Genetic polymorphism of the thiopurine S-methyltransferase of healthy Han Chinese]

BACKGROUND & OBJECTIVE

Several mutations in the thiopurine S-methyltransferase(TPMT) gene have been identified to correlate with a low activity or deficiency phenotype. The distribution of TPMT activity in Chinese and Caucasian populations is different, and the molecular basis has not yet to be determined. This study was designed to investigate the type and frequency of four kinds of mutant TPMT alleles of healthy Han Chinese.

METHODS

TPMT genotypes were determined in 225 healthy Han Chinese. Allele-specific PCR (ASPCR) and PCR-restriction fragment length polymorphism (RFLP) were used for analysis of G238C (TPMT*2), G460A and A719G (TPMT*3A,*3B,*3C).

RESULTS

TPMT*2, TPMT*3B, and TPMT*3A were not detected in 225 healthy Han Chinese, and only six TPMT*3C heterozygotes were identified. The average TPMT activity of those TPMT*3C heterozygotes was (9.62+/-3.58) U/ml RBC. The frequency of TPMT mutant alleles was 1.33%(6/450) in healthy Han Chinese.

CONCLUSION

The frequency of the known mutant TPMT alleles in Han Chinese population is low, and TPMT*3C appears to be the most prevalent among the known mutant TPMT allele in this population.

[Clinical analysis of 75 patients with nasopharyngeal non-Hodgkin's lymphoma]

BACKGROUND & OBJECTIVE

Nasopharynx is one of the common extranodal involved site in non-Hodgkin's lymphoma.However,no standard treatment regimen ever established for nasopharyngeal lymphoma(NPL). The purpose of this paper is to investigate the clinical characteristics and treatment results of NPL.

METHODS

The authors reviewed the records of 75 NPL patients who were managed from June 1976 to August 2001 in Cancer Center,Sun Yat-sen University,to evaluate the influence of clinical characteristics and treatment modality on survival.

RESULTS

Limited stage disease ( I/ II) was present in 90.8% of the NPL patients. The most common pathological type was intermediate grade(95.2%) and the most common immune phenotype was B-cell lineage (68.6%). These patients were treated with chemotherapy plus radiotherapy (64%),chemotherapy alone (23%),and radiotherapy alone (14%),respectively. The 2-, 5-, and 10-year overall survival rates were 79.1%, 69.8%, and 64.3%, respectively. For the patients who treated with stranded CHOP (cyclophosphamide + hydroxydaunomycin + Oncovin + prednisone) chemotherapy regimen. The 2-,5-,10-year overall survival rates were 84.6%. However, the patients treated with radiotherapy alone were associated with poor survival, 5 year survival was 0. There was no significant difference in the survival between patients treated with CHOP alone or CHOP plus radiotherapy. There was also no difference in the survival between radiotherapy dosage <or=50 Gy and >50 Gy.

CONCLUSION

The treatment modality for NPL should include systemic chemotherapy with CHOP regimen. Whether additional local radiotherapy is necessary,and the optimal radiation dosage warrant prospective randomized trial.

The distribution of stanniocalcin 1 protein in fetal mouse tissues suggests a role in bone and muscle development

We previously isolated a mammalian gene STC1 that encodes a glycoprotein related to stanniocalcin (STC), a fish hormone that plays a major role in calcium homeostasis. However, the mammalian STC1 gene is expressed in a variety of adult tissues in contrast to fish where STC is expressed only in one unique gland, the corpuscles of Stannius. This suggested that STC1 may have wider autocrine/paracrine functions in mammals. In the present study, using immunocytochemistry, we showed that STC1 protein is localized in the developing bone and muscle of the mouse fetus. During endochondral bone formation, STC1 is found principally in prechondrocytes and prehypertrophic chondrocytes. During intramembranous bone formation STC1 is present in the mesenchyme that is about to undergo ossification. STC1 is also found in the myocardiocytes of the developing heart and at all stages of differentiation from myoblasts to myotube formation in developing skeletal muscle. The specific localization of STC1 to chondrocytes and muscle cells suggests a role for this protein in chondrogenic and myogenic differentiation.

Altered distribution of the promyelocytic leukemia-associated protein is associated with cellular senescence

The disruption of the normal function and nuclear localization of the promyelocytic leukemia-associated protein (PML) may play a major role in the pathogenesis of acute promyelocytic leukemia. PML, which is concentrated in nuclear bodies (PML bodies), has been shown to have growth- and transformation-suppressive properties. In this study, we have examined the intranuclear distribution of PML in a conditionally immortalized human cell line (IDH4) in which both proliferation and immortalization are dependent on the presence of SV40-encoded large T-antigen (SV40T). Expression of SV40T is controlled by a dexamethasone (Dex)-inducible promotor. Suppression of SV40DT (Dex removal) in IDH4 cells causes G1 arrest and expression of the senescent phenotype. This is accompanied by a redistribution of PML in most cells from the usual pattern containing only spherical bodies to a pattern, containing large doughnut-like or fiber-like structures in addition to the spherical bodies. This change in pattern is reversed when phenotypically senescent IDH4 cells are stimulated to proliferate again by SV40T-induction. Moreover, we find that there is a similar change in the PML pattern between young and senescent or serum-starved young IMR90 human fibroblasts, from which IDH4 cells are derived. However, fewer serum-starved cells contain large PML bodies than senescent cells. Our observations suggest senescence, although it may be partly related to growth arrest. Using three-dimensional fluorescence digital imaging microscopy, we have found that the apparently doughnut-like PML structures have a cylindrical or egg-shaped form and that PML is concentrated to the outer shell of the structure.

Intranuclear redistribution of SV40T, p53, and PML in a conditionally SV40T-immortalized cell line

We have previously reported that EBNA-5, one of the Epstein-Barr virus-encoded proteins, accumulates in the nuclear bodies containing PML, the promyelocytic leukemia associated protein. In this study, we examine the intranuclear distribution of SV40 large T-antigen (SV40T), the p53 tumor suppressor protein (p53), and PML in a conditionally immortalized cell line, IDH4. In IDH4 cells, the expression of SV40T is regulated by a dexamethasone (Dex)-driven promoter. Withdrawal of Dex results in down-regulation of SV40T and growth arrest, whereas addition of Dex to the growth-arrested cells results in up-regulation of SV40T and proliferation. In proliferating IDH4 cells, SV40T is concentrated in nuclear dots that are also positive for p53. Many of these dots are juxtaposed to PML positive structures but do not colocalize with them. After removal of Dex, SV40T-p53 dots gradually disappear, while the PML structures remain. Induction of SV40T in nonproliferating IDH4 cells causes a coordinated redistribution of SV40T and p53. The immunostaining for SV40T and p53 is first weak, then strong with a homogeneous distribution, and 3-4 days later becomes dot-like again. This reappearance of SV40T-p53 dots coincides with the recovery of proliferation in restimulated IDH4 cells. Also, the p53 pattern correlates with the SV40T pattern with regard to both morphology and intensity during both suppression and induction of SV40T. Taken together, our data suggest that (i) the level of p53 is coregulated with the level of SV40T in a dose-dependent fashion; (ii) the formation of SV40T-p53 nuclear dots correlates with the transformed phenotype; (iii) the SV40T-p53 dots localize preferentially to the neighborhood of PML bodies which are already present in normal cells.

Processing of pre-mRNA in polytene nuclei of Chironomus tentans salivary gland cells

Salivary gland polytene cells in the dipteran Chironomus tentans provide exceptional experimental possibilities to analyze processing of specific pre-mRNAs in intact eukaryotic cell nuclei. Here we give a brief account of how these experimental advantages can be exploited to analyze the splicing process in vivo. In multi-intron pre-mRNAs, spliceosomes assemble and splicing is initiated cotranscriptionally for all introns. Intron excision may, however, occur mainly co- transcriptionally or mainly posttranscriptionally depending on the position of each intron in relation to the remaining transcription time and intron-specific efficiencies of excision. As measured for the U2 snRNP and an SR protein, 10-15% of the spliceosomal components are bound to pre-mRNA at active gene loci at a given moment, while the majority of the spliceosomal components are present in the nucleoplasm. A continuous redistribution of the spliceosomal components takes place in the nucleus as a result of a close coupling between transcription and spliceosomal assembly.

Demonstration of a dynamic, transcription-dependent organization of pre-mRNA splicing factors in polytene nuclei

We describe the dynamic organization of pre-mRNA splicing factors in the intact polytene nuclei of the dipteran Chironomus tentans. The snRNPs and an SR non-snRNP splicing factor are present in excess, mainly distributed throughout the interchromatin. Approximately 10% of the U2 snRNP and an SR non-snRNP splicing factor are associated with the chromosomes, highly enriched in active gene loci where they are bound to RNA. We demonstrate that the splicing factors are specifically recruited to a defined gene upon induction of transcription during physiological conditions. Concomitantly, the splicing factors leave gene loci in which transcription is turned off. We also demonstrated that upon general transcription inhibition, the splicing factors redistribute from active gene loci to the interchromatin. Our findings demonstrate the dynamic intranuclear organization of splicing factors and a tight linkage between transcription and the intranuclear organization of the splicing machinery.

Formation of nuclear bodies in cells overexpressing the nuclear pore protein POM121

POM121 is an integral membrane protein specifically localized in the pore membrane domain of the nuclear envelope. We have investigated the intracellular distribution of rat POM121 heterologously overexpressed in monkey COS cells by immunofluorescence and fluorescence digital imaging microscopy. At low levels of expression overexpressed POM121 was distributed in the nuclear envelope in a punctate fashion, partially overlapping with the distribution of nuclear pores. At high levels of expression, however, the overexpressed protein accumulated in intranuclear bodies. These bodies represent a novel subnuclear structure, displaying a defined cylindrical structure and a distinct localization at or adjacent to the inner nuclear membrane. The C-terminal portion of POM121, which contains a pentapeptide repeat domain common to a subfamily of related nucleoporins, was sufficient to mediate targeting to the nuclear envelope as well as formation of intranuclear bodies.

The Epstein-Barr virus-encoded nuclear antigen EBNA-5 accumulates in PML-containing bodies

EBNA-5 is one of the Epstein-Barr virus (EBV)-encoded nuclear proteins required for immortalization of human B lymphocytes. In the nuclei of EBV-transformed lymphoblastoid cell lines EBNA-5 is preferentially targetted to distinct nuclear foci. Previously we have shown (W.Q. Jiang, L. Szekely, V. Wendel-Hansen, N. Ringertz, G. Klein, and A. Rosen, Exp. Cell Res. 197:314-318, 1991) that the same foci also contained the retinoblastoma (Rb) protein. Using a similar double immunofluorescence technique, we now show that these foci colocalize with nuclear bodies positive for PML, the promyelocytic leukemia-associated protein. Artificial spreading of the chromatin by exposure to the forces of fluid surface tension disrupts this colocalization gradually, suggesting that the bodies consist of at least two subcomponents. Heat shock or metabolic stress induced by high cell density leads to the release of EBNA-5 from the PML-positive nuclear bodies and induces it to translocate to the nucleoli. In addition to their presence in nuclear bodies, both proteins are occasionally present in nuclear aggregates and doughnut-like structures in which PML is concentrated in an outer shell. Nuclear bodies with prominent PML staining are seen in resting B lymphocytes. This staining pattern does not change upon EBV infection. In freshly infected cells EBNA-5 antigens are first distributed throughout the nucleoplasm. After a few days intensely staining foci develop. These foci coincide with PML-positive nuclear bodies. At a later stage and in established lymphoblastoid cell lines EBNA-5 is almost exclusively present in the PML-positive nuclear foci. The colocalization is restricted to EBV-infected human lymphoblasts. The data presented indicate that the distinct EBNA-5 foci are not newly formed structures but the result of translocation of the viral protein to a specialized domain present already in the nuclei of uninfected cells.

Reversible nucleolar translocation of Epstein-Barr virus-encoded EBNA-5 and hsp70 proteins after exposure to heat shock or cell density congestion

The Epstein-Barr virus (EBV)-encoded, nuclear matrix-associated EBNA-5 protein is preferentially localized within distinct nuclear blobs in EBV-immortalized lymphoblastoid cell lines. We have previously found that the same blobs also contain retinoblastoma (Rb) protein. We now show that they contain hsp70 protein as well. Both EBNA-5 and hsp70 translocate to the nucleolus under cell density congestion or after heat shock. Both proteins relocate to their original position upon the re-establishment of normal physiological conditions. EBNA-5 is tightly bound to the nuclear matrix. The translocated EBNA-5 is also tightly associated with matrix structures, as shown by sequential elution-based cell fractionation. The Rb protein does not translocate to the nucleolus. The virally encoded EBNA-1, -2, -3 and -6, and cellular PCNA, snRNP and cyclin E are not affected either. The translocation of EBNA-5 to the nucleolus is not species- or cell type-specific since stress conditions induced the same phenomenon in EBNA-5-transfected human, mouse and rat cells of different tissue origins.

Resting B-cells, EBV-infected B-blasts and established lymphoblastoid cell lines differ in their Rb, p53 and EBNA-5 expression patterns

Using immunofluorescence technique we have analysed the Rb, p53, EBNA-2 and EBNA-5 expression pattern in EBV infected human B-cells and established lymphoblastoid cell lines (LCL-s). Resting B-cells showed only a faint Rb and no p53 immunostaining. The expression of both Rb and p53 increased after EBV infection. The change was first detectable 6 h after infection. The frequency of brilliantly Rb positive cells increased more rapidly than p53 positives. EBNA-2 and EBNA-5 became first detectable 12 h after infection. The frequency of EBNA positive cells in the freshly infected cultures was concordant with the proportion of CD23 and PCNA positives, but remained consistently below the frequency of Rb and p53 positive cells. Double immunofluorescence staining showed that all EBNA-5 positive cells were strongly Rb and p53 positive. LCL-s did not stain for p53, whereas the Rb staining was maintained at a high level. The EBNA-5 staining pattern changed from brilliant almost homogeneous nuclear staining in the freshly infected B-cells, to a nonhomogeneous pattern with a small number of strongly fluorescent nuclear bodies in established LCL-s. There was no change in the EBNA-2 staining pattern. Our findings indicate that the immortalization of B-cells by EBV may initially involve a high expression of EBNA-5, p53 and Rb, but only cells with low p53 and focal expression of EBNA-5 in nuclear bodies have the selective advantage required to grow into immortalized lines.

Colocalization of nestin and vimentin/desmin in skeletal muscle cells demonstrated by three-dimensional fluorescence digital imaging microscopy

During skeletal muscle development three intermediate filament proteins are expressed: nestin, vimentin, and desmin. Vimentin and desmin belong to the class III intermediate filaments and are closely related to each other, whereas nestin is a more distantly related, class VI, intermediate filament. It was previously observed by conventional immunocytochemistry that the intracellular patterns of nestin, desmin, and vimentin appeared indistinguishable, despite nestin's more distant evolutionary relationship. We here extend this analysis by applying three-dimensional fluorescence digital imaging microscopy to compare the intracellular distribution of nestin with that of desmin, vimentin, actin, and tubulin in G6 human fetal skeletal muscle cells. We show that in vitro differentiation of G6 cells can produce an intermediate filament expression pattern similar to that observed during myogenesis in vivo, i.e., downregulation of vimentin but not of nestin and desmin during myotube maturation. The image analysis demonstrated that the degree of overlap between nestin and desmin/vimentin was very extensive in myoblasts and in multinucleate myotubes in all regions of the cells. In contrast, nestin did not colocalize with tubulin or actin in G6 myoblasts. In particular, nestin immunoreactivity was not detected at the microtubule-organizing center, and it was only sparsely observed at the cell periphery where actin stress fibers were seen. Our data lend further support to the notion that nestin interacts very closely with the two more distantly related class III intermediate filament proteins desmin and vimentin in the entire muscle cell, before and after myotube formation. A comparison of conserved amino acid residues in the different IFs suggest that charged amino acid residues in the alpha-helical rod domain may play a role in the interaction.

Position-dependent nuclear accumulation of the retinoblastoma (RB) protein during in vitro myogenesis

The expression of the retinoblastoma (RB) protein has been studied during in vitro muscle differentiation by immunofluorescence staining with three different antibodies against RB protein. Proliferating mononucleate L6 rat myoblasts showed a low level of expression. As cells began to enter a nonreplicating G0 state, the cell population became heterogeneous. Some nonreplicating cells showed a high level of expression. Nuclei at the two ends of myotubes were strongly positive, whereas centrally located nuclei showed low RB expression. Overexpression of the human RB protein in rat L6 myotubes from a Semliki forest virus (SFV)-based, transient expression vector produced a similar picture. Terminally located nuclei expressed human RB at a much higher level than did the centrally located nuclei. The results suggest that individual nuclei with a multinucleated syncytium may undergo position-dependent specialization.

Endogenous activation of c-myc expression and DNA synthesis in serum-starved neonatal rat smooth muscle cells

Earlier studies have shown that smooth muscle cells (SMC) from arteries of neonatal and adult rats differ markedly in their in vitro growth characteristics. Since some of these differences may be relevant to the proliferation of SMC in atherosclerotic plaques we examined the expression of three proto-oncogenes (c-fos, c-jun, and c-myc) and an SMC-specific differentiation marker (alpha-actin) in cultured SMC. In presence of serum cultured adult SMC contained lower levels of alpha-actin mRNA than neonatal cells. In neonatal cells serum-starvation resulted in a distinct increase in both c-myc and alpha-actin mRNA levels, whereas the expression of these genes appeared to be unaffected in adult cells. Transfer of adult SMC proliferating in the presence of fetal calf serum to serum-free medium for 48 h almost completely inhibited DNA synthesis, whereas transfer of neonatal SMC to serum-free medium reduced DNA synthesis only to about 50%. Serum-starved adult and neonatal SMC did not contain c-fos or c-jun transcripts, but in both cell types serum-stimulation resulted in a comparable increase in the expression of both genes. The present results demonstrate clear differences in the mechanisms regulating gene expression in adult and neonatal SMC.

Cell type and differentiation dependent heterogeneity in retinoblastoma protein expression in SCID mouse fetuses

The expression pattern of retinoblastoma (Rb) protein has been studied at the single cell level in frozen sections of 16- to 18-day-old SCID mouse fetuses by immunofluorescence staining with mouse monoclonal and rabbit polyclonal antibodies, using conventional epifluorescence and confocal laser scanning microscopy. The nuclei of megakaryocytes, hemopoietic islands of the fetal liver, osteo-, amelo-, and odontoblasts, and skeletal muscle were strongly stained. There was no detectable Rb staining in the basal cell layers of stratified squamous epithelia, but the differentiating, more superficial layers were positive. Intestinal crypts were negative, whereas the villi were positive. In the retina, Rb protein was detectable in the inner ganglion layer but not in the outer neuroblastic layer. In the central nervous system, Rb protein was present in neurons and glia cells as well. The nuclei in the collecting tubules of the kidney, the pancreas, and the adrenal cortex were Rb positive. Analysis of the differentiation dependent expression of Rb protein in relation to the prospective life cycle of the cells in which it appears may pave the way toward an understanding of the tissue specific oncogenic effect of Rb loss in families with hereditary retinoblastoma.

Intranuclear localization of a new snRNP-related antigen

The intranuclear distribution of a new antigen (F78) associated with U snRNPs (small nuclear RNA-protein complexes) was compared with that of the RNP and Sm protein antigens previously identified on individual snRNP particles. Human and rat cells were double stained with human autoantisera and mouse monoclonal antibodies. The binding of the human and mouse antibodies was detected with secondary antibodies conjugated with fluorescein and rhodamine, respectively. The resulting immunofluorescence patterns were compared by digital image analysis. The F78, RNP, and Sm antigens show speckled fluorescence patterns which overlap to a great extent. The F78 pattern, however, also contains two classes of structural elements not present in the RNP pattern. Furthermore, during mitosis expression of the F78 antigen is completely suppressed from early prophase to telophase, while the RNP and Sm antigens are found evenly distributed throughout the cytoplasm of the dividing cells.

Co-localization of the retinoblastoma protein and the Epstein-Barr virus-encoded nuclear antigen EBNA-5

A monoclonal antibody (aRB1C1) raised against an Rb fusion protein detects a limited number (4-10) of relatively large intranuclear foci in an EBV-immortalized cord blood cell line (IB4). These domains also bind an anti-EBNA-5 monoclonal antibody. The Rb antibody reactive sites also co-localize with the SV40 T antigen in transformed monkey cells (COS). The nuclear structures stained by aRB1C1 and EBNA-5 antibodies are distinct from the structures detected with antibodies against centromeric proteins and certain snRNP epitopes. EBNA-5/Rb-positive domains do not selectively react with antibodies against the La antigen known to associate with the small EBV-encoded nuclear RNA species designated as the EBERs.

Intranuclear distribution of Epstein-Barr virus-encoded nuclear antigens EBNA-1, -2, -3 and -5

Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs) express at least seven virally encoded proteins. Their functional role, and their relationships to each other and to normal nuclear constituents are virtually unknown. As the first step towards a topographical study, the intranuclear distribution of EBV-encoded nuclear antigens EBNA-1, -2, -3 and -5 (abbreviated E1, E2 etc.) was examined in EBV-transformed LCLs by immunofluorescence and digital image analysis of fluorescence patterns. E1-E3 showed a finely granular distribution. The E2 patterns were virtually identical when comparing indirect staining using an E2-specific mouse monoclonal antibody with anticomplement immunofluorescence using a human antibody, rendered monospecific to E2 by absorption. The E1/E2 patterns showed 32% overlap and the E2/E3 10% overlap in the high overlap category (66.7-100%), while the E2/E2 comparison with two reagents showed 61% overlap in this category. This suggests that E2 and E3 largely appear in different nuclear structures, whereas E1 appears to be randomly distributed with regard to E2. The E5 pattern was radically different from that of E1, E2 and E3. The anti-E5 mouse monoclonal antibody detected 4–10 huge, globular, sharply circumscribed dots, located in dispersed chromatin areas, while the distribution of E1, E2 and E3 showed no obvious relationship to chromatin distribution. The methods described here allow a more refined topographical analysis of the EBNA protein family, mostly in relation to each other, in relation to other nuclear proteins, and with respect to specialized functional domains in interphase chromatin.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular distribution of the c-fos antigen during the cell cycle

The subcellular localization of the c-fos proto-oncogene product was studied in the G1, S, G2, and mitotic phases of the cell cycle by indirect immunofluorescence. For these analyses c-fos transfected L6J1 rat skeletal myoblasts and adult rat aortic smooth muscle cells in secondary culture, and c-fos- and c-myc co-transfected mouse Swiss 3T3 fibroblasts were used. During G1, S, and G2, the c-fos protein was evenly distributed in the nucleus, with exclusion of the nucleoli. In mitotic prophase the c-fos antigen was dissociated from the condensed chromosomes and became diffusely distributed in the cell cytoplasm, where it remained until telophase, when, again, it appeared to be associated with chromatin in the re-assembling nucleus. When comparing the subnuclear distribution of the c-fos product with that of densely packed DNA, stained with the fluorochrome Hoechst, an inverse relationship was found. Dispersed chromatin regions with weak Hoechst DNA fluorescence showed a stronger fos immunofluorescence than regions that contained a higher concentration of DNA. The localization of c-fos antigen partially overlapped with that of antigens typical of small nuclear ribonucleoprotein complexes participating in transcription and splicing. To examine if the c-fos protein would bind preferentially to specific interphase chromosomes the nucleus was fragmented into micronuclei containing single, or groups of, chromosomes. Immunofluorescence analysis showed that the majority of micronuclei were fos-positive. Possible roles of the c-fos proto-oncogene product are discussed in relation to other nuclear antigens.