The M locus and ethylene-controlled sex determination in andromonoecious cucumber plants

Sex determination in cucumber (Cucumis sativus L.) plants is genetically controlled by the F and M loci. These loci interact to produce three different sexual phenotypes: gynoecious (M-F-), monoecious (M-ff), and andromonoecious (mmff). Gynoecious cucumber plants produce more ethylene than do monoecious plants. We found that the levels of ethylene production and the accumulation of CS-ACS2 mRNA in andromonoecious cucumber plants did not differ from those in monoecious plants and were lower than the levels measured in gynoecious plants. Ethylene inhibited stamen development in gynoecious cucumbers but not in andromonoecious ones. Furthermore, ethylene caused substantial increases in the accumulation of CS-ETR2, CS-ERS, and CS-ACS2 mRNA in monoecious and gynoecious cucumber plants, but not in andromonoecious one. In addition, the inhibitory effect of ethylene on hypocotyl elongation in andromonoecious cucumber plants was less than that in monoecious and gynoecious plants. These results suggest that ethylene responses in andromonoecious cucumber plants are reduced from those in monoecious and gynoecious plants. This is the first evidence that ethylene signals may influence the product of the M locus and thus inhibit stamen development in cucumber. The andromonoecious line provides novel material for studying the function of the M locus during sex determination in flowering cucumbers.

Combined immunodeficiency, chromosomal instability, and postnatal growth deficiency in a Japanese girl

We report on an 11-year-old Japanese girl with combined immunodeficiency and chromosomal instability. She had postnatal growth deficiency and microcephaly, preaxial polydactyly of the left hand, and susceptibility to infections. Immunological studies showed marked lymphocytopenia (around 500/ll), reduced lymphocyte response to various mitogens, and reduced or absent serum IgA, IgG, and IgM. Cell biological studies of her primary skin fibroblasts demonstrated spontaneous chromosome aberrations and radiation hypersensitivity. The combination of immunodeficiency, chromosomal instability, and radiation hypersensitivity as seen in the girl is present in both ataxia-telangiectasia and Nijmegen breakage syndrome. Ataxia-telangiectasia was excluded because of differences in clinical features and laboratory data. Likewise, Nijmegen breakage syndrome is unlikely to be the case because the characteristic face, hyperpigmented spots, and mental retardation present in the syndrome were missing in the girl. Sequence analysis of a Nijmegen breakage syndrome responsible gene, NBS1, revealed no mutations. A normal NBS1 product was also demonstrated by immunoblot analysis using an anti-NBS1 antibody. We propose that the disorder in the girl represents a new combination of combined immunodeficiency and chromosomal instability.

Micturition evoked by glutamate microinjection in the ventrolateral periaqueductal gray is mediated through Barrington's nucleus in the rat

Neural tracing experiments have demonstrated a direct spinal projection to Barrington's nucleus and a possible indirect pathway to Barrington's nucleus via the periaqueductal gray. We sought to identify the role of the periaqueductal gray matter in micturition in urethane-anesthetized rats. Blockade of micturition by focal injection of cobalt chloride was used to identify sites critical to micturition. These sites were located near the ventral margin of the caudal ventrolateral periaqueductal gray and in Barrington's nucleus. L-Glutamate injections into caudal regions of the periaqueductal gray evoked bladder contraction with coordinated sphincter activation. Additional L-glutamate sites with a similar pattern of response and sites where sphincter activation was produced without bladder contraction were found more rostrally and dorsally in the periaqueductal gray. Activation of bladder contractions by L-glutamate injection in the ventrolateral periaqueductal gray was blocked by prior injection of cobalt chloride into Barrington's nucleus. From these data we propose that ventrolateral periaqueductal gray is functionally important to micturition in the urethane-anesthetized rat. Further, we have shown that a periaqueductal gray to Barrington's nucleus pathway is functionally relevant to central mediation of bladder contraction.

Characterization of monoclonal antibodies generated against Norwalk virus GII capsid protein expressed in Escherichia coli

The Norwalk virus (NV) causes outbreaks of acute non-bacterial gastroenteritis in humans. The virus capsid is composed of a single 60 kDa protein. The capsid protein of NV36 (genogroup II, Mexico virus type) was expressed in an Escherichia coli system and ten monoclonal antibodies (MAbs) were generated against it. The reactivity of these MAbs was characterized using enzyme-linked immunosorbent assay (ELISA) and Western blot (WB) analysis towards 20 overlapping fragments of the NV36 capsid protein expressed in E. coli. All of the MAbs recognized sequential (continuous) epitopes on the three antigenic regions. Six of the 10 MAbs recognized fragment 2 (equivalent residues 31-70), three MAbs recognized fragment 13 (residues 361-403) and one MAb recognized fragment 7 (residues 181-220), suggesting that the N-terminal domain (residues 1-220) may contain more antigenic epitopes than the C-terminal domain (residues 210-548). Furthermore, two MAbs (1B4 and 1F6) reacted in WB with three purified NV strains (genogroup II) derived from patients' stool samples. It was also found that genogroup I recombinant NV96-908 (genogroup I, KY89 type) could be detected as sensitively as recombinant NV36 (genogroup II) by ELISA with a set of the MAbs produced here.

Structural characteristics of the N-glycans of two isoforms of prostate-specific antigens purified from human seminal fluid

Prostate-specific antigen (PSA) is a glycosylated chymotrypsin-like serine protease and is found mainly in prostatic tissue and seminal fluid. We purified two forms of PSA (PSA-A and PSA-B) from human seminal fluid with pI values of approx. 7.2 and approx. 6.9, respectively. To characterize the N-glycans of the two isoforms, the sugar chains were liberated by hydrazinolysis followed by N-acetylation, and derivatized with 2-aminobenzamide. Both PSA-A and PSA-B contained mono- and disialylated sugar chains, although PSA-B had a much higher content of the latter. After removal of sialic acid residues by sialidase digestion, mono- and biantennary N-glycans and three outer chain moieties (Galbeta1-4GlcNAcbeta1-, GlcNAcbeta1-, GalNAcbeta1-4GlcNAcbeta1-) were found in both samples. However, the ratios of each N-glycan were different. These results indicate that PSA-A and PSA-B differ not only in their sialic acid contents, but also in their outer chain features.

The forkhead-associated domain of NBS1 is essential for nuclear foci formation after irradiation but not essential for hRAD50[middle dot]hMRE11[middle dot]NBS1 complex DNA repair activity

NBS1 (p95), the protein responsible for Nijmegen breakage syndrome, shows a weak homology to the yeast Xrs2 protein at the N terminus region, known as the forkhead-associated (FHA) domain and the BRCA1 C terminus domain. The protein interacts with hMRE11 to form a complex with a nuclease activity for initiation of both nonhomologous end joining and homologous recombination. Here, we show in vivo direct evidence that NBS1 recruits the hMRE11 nuclease complex into the cell nucleus and leads to the formation of foci by utilizing different functions from several domains. The amino acid sequence at 665-693 on the C terminus of NBS1, where a novel identical sequence with yeast Xrs2 protein was found, is essential for hMRE11 binding. The hMRE11-binding region is necessary for both nuclear localization of the complex and for cellular radiation resistance. On the other hand, the FHA domain regulates nuclear foci formation of the multiprotein complex in response to DNA damage but is not essential for nuclear transportation of the complex and radiation resistance. Because the FHA/BRCA1 C terminus domain is widely conserved in eukaryotic nuclear proteins related to the cell cycle, gene regulation, and DNA repair, the foci formation could be associated with many phenotypes of Nijmegen breakage syndrome other than radiation sensitivity.

Indirect micromanipulation of single molecules in water-in-oil emulsion

Based on real-time observation and micromanipulation, analytical methods for single DNA molecules have been under development for some time. Precise manipulation, however, is still difficult because single molecules are too small for conventional techniques. We have developed a chemical reaction system that uses water droplets in oil as containers of materials. The water droplets can be manipulated by optical force. The manipulation of the water droplets permits the fusion of two selected droplets. This process corresponds to mixing of different samples. We designate this system as "w/o (water-in-oil emulsion) microreactor system", and each droplet can be thought of as a "microreactor". In this system, single molecules can be manipulated readily, as a molecule can be contained in a microm-sized microreactor. The microreactor utilizes extremely small quantities of samples, therefore, reactions are rapid, as diffusion times in the microreactor are very short. The manipulation technique of the microreactors based on optical force has been applied to induce fusion between microreactors loaded with DNA and YOYO, a fluorescent dye that binds to DNA. This fusion induced a rapid binding of YOYO.

Comparative genomic hybridization analysis suggests a gain of chromosome 7p associated with lymph node metastasis in non-small cell lung cancer

We analyzed the chromosomal gains and losses that occur in 30 non-small cell lung carcinomas by comparative genomic hybridization. Their chromosomal imbalances showed histological type-specific patterns in adenocarcinomas and in squamous cell carcinomas. The genetic changes in non-small cell lung carcinoma were also strongly dependent on metastasis to lymph node. The average numbers of chromosomal alterations were increased from 6.2 to 9.1 along with the presence of metastasis, and it gave rise to the increased copy number in specific chromosomes. In particular, a novel imbalance at 7p12-21 was recognized in a half of carcinoma with metastasis, although no genetic alteration was observed in 15 non-metastasizing lung carcinoma tested here.

Expression of kininogens in the connective tissue-type mast cells of the rat

The connective tissue-type mast cells present in the submandibular gland (SMG) and peritoneal cavity of rats were found to express kininogens (KGs), the expression of which was demonstrated by Western blotting, reverse transcription-polymerase chain reaction (RT-PCR), RT-PCR Southern blotting, and light- and electron-microscopic immunocytochemistry. In the SMG, the analysis of cDNA amplified by RT-PCR revealed that the molecular species of mRNAs expressed were high-molecular-weight (HMW)-K KG and T-I KG. Light microscopic immunocytochemistry exclusively localized the KG protein(s) in the mast cells present in the SMG. The signals in the mast cells were very strong, but no positive reaction was observed in the granular convoluted tubular cells, acinar cells or striated duct cells. As determined by using electron microscopy, extremely strong labelling with immunogold was observed in the secretory granules of the mast cells, but no labelling in their nucleus or cytoplasm. Analysis by Western blotting and RT-PCR Southern blotting indicated that both protein and mRNA of KGs were present in the mast cells separated from the peritoneal cavity, indicating de novo synthesis of KG in these cells. Preliminary experiments implied that the connective tissue-type mast cells in other rat tissues also expressed KG.

Effect of selective IL-6 inhibitor Am-80 on experimental autoimmune encephalomyelitis in DA rats

AIM

To observe the role of interleukin (IL)-6 in the development of experimental autoimmune encephalomyelitis (EAE).

METHODS

DA rats were immunized by injecting bovine myelin basic protein (MBP). mRNA of cytokines, such as IL-6, IL-10, TNF-alpha, TGF-beta 1, IFN-gamma, and iNOS, were detected by RT-PCR. MBP was injected into ear to induce delayed type cutaneous hypersensitivity response (DTH). Histological studies were performed on the spinal cord with HE staining. Nitric oxide (NO) production from cultured murine macrophage clones was stimulated with LPS plus IFN-gamma.

RESULTS

DA rats developed EAE disease with a peak of severity on d 13 and d 14. Am-80 (1.0, 3.0 mg/kg), a selective IL-6 inhibitor, inhibited the symptoms in terms of deterioration as observed by the clinical score, body weight and histological findings, in a dose-related manner. A high dose of Am-80 (3.0 mg/kg for 12 d) did not completely inhibit the disease, but delayed the symptoms and enhanced the delayed response. By prolonging the duration of treatment (18 d), Am-80 inhibited the onset of EAE during administration, but the symptoms of EAE appeared after the administration was stopped. Am-80 administerd for 12 d inhibited the DTH response on d 11 but not on d 22. RT-PCR studies demonstrated a strong expression of IFN-gamma, IL-6, IL-10, TGF-beta 1, TNF-alpha, and iNOS mRNA in spinal cord 13 d after immunization. However IFN-gamma, IL-10, TNF-alpha, and iNOS mRNA expression (on d 13) was suppressed by Am-80, except in the case of IL-6, hence the effect of Am-80 on the expression of IL-6 mRNA was examined in additional experiments. After Am-80 was administered for 12 d or 18 d, the expression of IL-6 mRNA was inhibited on d 12 or d 18, but increased on d 13 or d 19, respectively.

CONCLUSION

These findings suggest that inhibition of EAE by Am-80 is initiated by inhibition of IL-6 production.

Japanese family with an autosomal dominant chromosome instability syndrome: a new neurodegenerative disease?

We report on a Japanese family having an autosomal dominant neurodegenerative disease with chromosomal instability and radiosensitivity. Clinical manifestations of affected members included short stature, osteoporosis, severe dental caries, and various neurological abnormalities, such as mental retardation, depression, dysarthria, hyperreflexia, and ataxic gait. MRI demonstrated a markedly atrophic spinal cord and degeneration of the white matter. Cytogenetic examination showed spontaneous chromosome rearrangements at 14q11.2 and hypersensitivity to radiation and bleomycin. The degree of these cytogenetic abnormalities was significantly higher in the patients than in normal controls but lower than in patients with ataxia telangiectasia or Nijmegen breakage syndrome. Moreover, genetic anticipation was observed in this family: the age of disease onset became earlier, MRI abnormalities more extensive, and the chromosome hypersensitivity to radiation increased in successive generations. We speculate that a basic defect in this family is a mutation in the gene that is responsible for DNA double-strand breakage repair.

The effect of Am-80, one of retinoids derivatives on experimental allergic encephalomyelitis in rats

Experimental allergic encephalomyelitis (EAE) is an autoimmune model with inflammation and demyelination in the central nervous system, which resembles the human demyelinating disorder, multiple sclerosis (MS). In this study, we investigated the effect of Am-80, a synthetic retinoid, on EAE in DA rats. DA rats immunized with complete Freund's adjuvant (CFA) supplemented with myelin basic protein (MBP) developed severe EAE which reached the peak 12 to 14 days after immunization. Am-80 and prednisolone administered orally for 12 days after immunization diminished the clinical symptoms and infiltration of inflammatory cells in a dose dependent manner. However, after stopping administration, EAE recurred in DA rats treated with Am-80, but not with prednisolone. The different responses between Am-80 and prednisolone were not due to the difference in the tolerability to the MBP because both inhibited the delayed-type hypersensitivity response to MBP only during administration. To investigate the mechanism how Am-80 alone delayed the response, the expressional levels of mRNA for interleukin-6 (IL-6), interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) in spinal cord were examined. Transcriptional levels of IL-6, IFN-gamma and TNF-alpha were parallel with the clinical symptoms of the disease in Am-80-treated rats, that is, expressional levels of their mRNA were diminished during the administration of Am-80, which then increased as soon as the administration was stopped. Among them, the expression of IL-6 mRNA was more rapidly and highly relapsed than that of the other two cytokines mRNA. However, prednisolone attenuated transcriptions of all these cytokines throughout the experiment. Therefore, these findings suggested that the inhibition of EAE is, in part, related to the inhibition of IL-6 production. However, there are many possible mechanism in the suppression of EAE by Am-80, further experiments will be necessary.

Listeriosis in a raccoon dog (Nyctereutes procyonoides) associated with canine distemper

A wild raccoon dog (Nyctereutes procyonoides) that manifested severe illness and died was examined. Necropsy revealed severe emaciation, systemic icterus and petechial hemorrhages on the mucous membranes. Histopathologically, necroses were seen in the liver and brain stem associated with meningitis. Eosinophilic intranuclear inclusion bodies were observed in the spleen and intestinal mucosa, and eosinophilic intracytoplasmic inclusion bodies were seen in transitional epithelium in the bladder. Listeria monocytogenes 4b was isolated from the liver, spleen, kidneys and lungs, and the pathogen was also detected in the liver and brain stem immunohistopathologically. The disease was diagnosed as listeriosis associated with canine distemper virus infection in a raccoon dog.

Absence of mutations in the NBS1 gene in B-cell malignant lymphoma patients

BACKGROUND

Nijmegen breakage syndrome (NBS), also known as ataxia-telangiectasia (AT) variant, is an autosomal recessive disorder characterized by microcephaly, growth retardation, severe combined immunodeficiency and a high incidence of lymphoid carcinoma, the majority of which are B-cell lymphomas. To determine whether the NBS1 gene is a tumor suppressor gene in B-cell lymphoma, we screened B-cell malignant lymphoma (ML) for any evidence of NBS1 mutation.

MATERIALS AND METHODS

Sequence analysis of the NBS1 gene was performed from PCR products amplified from the DNA of 12 extracranial ML or RT-PCR products amplified from cDNA of 8 primary central nervous system lymphoma.

RESULTS

Direct sequence analysis revealed that no NBS1 mutations were present in any of these patients.

CONCLUSION

The present results suggested that the contribution of NBS1 mutations to B-cell ML was minimal, despite the fact that the NBS1 gene was causative factor in these cases.

Trapezoid bone fracture

Fractures of the carpal bones involve only a single bone or complex bones with or without ligament rupture. However, fractures of the trapezoid are rarely seen. Because the trapezoid is fastened to the trapezium, capitate, and scaphoid by strong ligaments, fracture or dislocation is limited by this rigid fixation. The authors present a single bone fracture of the trapezoid in a 40-year-old man. A tomogram of the carpal bone was useful in diagnosing the trapezoid fracture. The mechanism for development of fracture of the trapezoid alone is unknown. However, fracture of the trapezoid seemed to occur when the wrist joint was forced with excessive flexion stress that was placed on the trapezoid through the second metacarpal bone indirectly. This occurred in the same manner that a walnut is broken with nutcrackers.

Four novel mutations of the Fanconi anemia group A gene (FAA) in Japanese patients

Fanconi anemia (FA) is an autosomal recessive disorder characterized by pancytopenia, predisposition to cancers, and a diverse variety of congenital malformations. At least eight complementation groups, A through H, have been described. Recently, the FA-A gene (FAA) has been isolated, and a large number of distinct mutations reported in ethnically diverse FA-A patients. Here, we report on the mutation analysis of five FA patients by single-strand conformation polymorphism. Out of five patients, at least three were found to have mutations in the FAA gene. The first patient was a compound heterozygote with a 1-bp deletion and a single-base substitution. The second patient had a heterozygous 2-bp deletion, which introduces a premature termination codon, and the third patient had a heterozygous splice donor site mutation in intron 27.

Retinopathy in a patient with thrombotic thrombocytopenic purpura complicated by polymyositis

BACKGROUND

Cotton wool spots are known to be a complication in patients with thrombotic thrombocytopenic purpura or with polymyositis.

CASE

A 53-year-old woman developed numerous cotton wool spots around the optic disc of both eyes.

OBSERVATIONS

Fluorescein angiography disclosed capillary obstruction and microaneurysms in the early phase, followed by dye leakage in the late phase. Systemically, she showed fever, hemolytic anemia, fragmented red blood cells, thrombocytopenia, proximal muscle weakness, and elevation of serum muscle enzymes. This led to a diagnosis of thrombotic thrombocytopenic purpura complicated by polymyositis.

CONCLUSION

We believe that this is the first case of thrombotic thrombocytopenic purpura complicated by polymyositis to develop cotton wool spots.

An occurrence of Salmonella typhimurium infection in sika deer (Cervus nippon)

Seven sika deer (Cervus nippon) in a herd of 30 deer in a park died. Upon examination of three dead deer, Salmonella Typhimurium was isolated from the organs and intestinal contents. Histopathological examination revealed catarrhal enteritis and focal necroses in the liver. Immunohistochemically, Salmonella antigen of O4 was detected in the enteric lesions. The case was diagnosed as S. Typhimurium infection in the sika deer. Because of the importance of Salmonella in public health, fecal and soil samples were continuously collected from the paddock. However, no Salmonella was isolated from any samples collected after medication of the deer and thorough disinfection of the immediate environment.

The effect of modified ultrafiltration in pediatric open heart surgery

Since 1997 we have performed modified ultrafiltration (MUF) in pediatric open heart operations. To elucidate the clinical effects of MUF, patients under 20 kg in weight who underwent corrective open heart operation since 1997 are divided into 2 cohorts according to the enforcement of MUF (control group versus MUF group) retrospectively. Procedures, age, bodyweight, cardiopulmonary bypass (CPB) time, operation time, amount of donor blood use, postoperative inotrope dose, postoperative intubation time, and postoperative gas exchange capacity of lung expressed with respiratory index (RI) were compared between groups. Furthermore, a multiple linear regression analysis was performed to find independent correlates with postoperative RI. Finally scattergrams of intubation time and RI were drawn against several factors. In univariate analyses, the MUF group had significantly shorter intubation time and better RI. Multivariate analysis revealed that the enforcement of MUF was an independent correlate of postoperative RI. Analyses on the scattergrams revealed that the above mentioned favorable effects of MUF were prominent in the patients younger than 3 years or weighing less than 10 kg. We concluded that MUF had significant effects on pulmonary function preservation in pediatric open heart operations, especially for smaller children.

Application of telepsychiatry: a preliminary study

Telemedicine is the provision of professional medical services from sites removed from the patient. This study was conducted in order to assess the reliability of psychiatric evaluations via video-consultation. The inter-rater reliability was determined for each interviewing condition by employing the reliability-testing paradigm in which the brief psychiatric rating scale (BPRS) and the interclass correlation coefficient (ICC) are used. The inter-rater reliability scores between face-to-face interviews and interviews conducted by video at two different levels of image qualities were compared by the authors. Reliability was found to be in perfect agreement in each of the interviewing conditions. This study verifies that telepsychiatry is a reliable method of assessing patient conditions. It may be used to provide much needed mental health-care services to patients whose access to them is restricted.

Urinary disturbance after therapy for cervical cancer: urodynamic evaluation and beta2-agonist medication

Urinary disturbance frequently develops following therapy for cervical cancer; however, no effective medical treatment has so far been reported. Sixty-five patients who developed urinary disturbance after radiation therapy, radical hysterectomy or radical hysterectomy with radiation therapy for cervical cancer underwent urodynamic assessment. Those who underwent radical hysterectomy with radiation therapy experienced the most severe urine loss, as determined by the pad test. All patients showed markedly reduced bladder compliance. A beta2-agonist (mabuterol) significantly improved compliance, bladder capacity and flow rate. It is suggested that medication with mabuterol is a potential novel approach to the treatment of urinary disturbance after therapy for cervical cancer.

Effect of anesthetics on reflex micturition in the chronic cannula-implanted rat

It is well known that urethane is a suitable anesthetic for acute studies and has been extensively recommended for investigations related to micturition physiology. This is mainly because of the capability of urethane anesthesia to spare reflex micturition as well as its easily established long-lasting and stable anesthetic level. However, urethane anesthesia is usually restricted to acute experiments due to its potential toxicity. This study searched for an alternative to urethane that would be suitable for studies in which recovery from anesthesia was needed. The list of administered drugs was as follows: pentobarbital, thiobutabarbital, ketamine-acepromazine, ketamine-diazepam, tiletamine-zolazepam, fentanyl-droperidol, alphaxalone-alphadolone, propofol, isoflurane, methoxyflurane, azaperone, tribromoethanol, and buprenorphine. Among these drugs, only tiletamine-zolazepam spared the reflex micturition contractions. However, the duration of this anesthesia was too short (approximately 30 minutes) to complete the necessary testing and additional dosing of the anesthetic generally obliterated the micturition reflex. On the other hand, rats given i.v. urethane infusion (10% solution in 0.9% saline, 3.2-4.0 mg/kg/min, total dose 0.56-1.03 g/kg) maintained a stable anesthesia that permitted both reflex micturition and stereotaxic procedures. Rats moved spontaneously 3-16 hours after cessation of i.v. urethane anesthesia and completely recovered in 2 days without significant after-effects. Bladder function was normal. No pathological changes were seen 1 week later. The present results suggest that urethane is the most suitable anesthetic for acute and chronic physiological experiments that require demonstration of reflex micturition. Neurourol. Urodynam. 19:87-99, 2000.

Cell cycle and LET dependence for radiation-induced mutation: a possible mechanism for reversed dose-rate effect

A previous study of the mutagenic action of 252Cf radiation in mouse L5178Y cells showed that the mutation frequency was higher when the dose was chronic rather than acute, which was in sharp contrast to the effects reported for gamma-rays (Nakamura and Sawada, 1988). A subsequent study using synchronized cells revealed that the cells at the G2/M stage were uniquely sensitive to mutation induction by 252Cf radiation but not to gamma-rays (Tauchi et al., 1993). A long phase cell population was first subjected to conditioning gamma or 252Cf radiation doses at different dose-rates. The cell cycle distribution of these cells was then observed, and they were then exposed to 252Cf radiation, and the mutation rate was determined. The G2/M fraction increased by 3- to 4-fold when the conditioning doses (2 Gy of gamma or 1 Gy of 252Cf radiation) were delivered chronically over 10 h, but only slightly when the same doses were delivered over a 1 h period or less. Subsequent 252Cf irradiation gave higher mutation frequencies in the cells pre-irradiated with gamma-rays over a protracted period of time than in those exposed with the higher dose-rate gamma-rays. These results suggest that the radiation-induced G2 block could be at least partly (but not totally) responsible for this reverse dose-rate effect (Tauchi et al. 1996). Possible factors which cause the hyper-sensitivity of G2/M cells to mutation induction by neutrons will be discussed.