In order to evaluate the relation between pericentric inversion of chromosome 9 (inv (9)) and clinical problems, the characteristics of inv (9) were investigated on the basis of chromosomal analyses of fetuses and infertile couples. The incidence of such inversion in fetuses with parents having an offspring who suffered from various clinical problems was significantly higher than the basic incidence obtained in fetuses karyotyped by reason of advanced maternal age. In the chromosomal examination of the parents whose fetuses were diagnosed as inv (9), it was revealed that either parent might be the carrier. Furthermore, in the inv (9) carrying fetuses, the number of females was significantly greater than that of males. Analysis of infertile couples revealed that the incidence of such inversion in males was significantly higher than the basic incidence mentioned above. Moreover, infertile couples with an inv(9) carrier showed a significantly higher incidence of intrauterine fetal death, compared with infertile couples with a translocation carrier or those in which the etiology was unknown. These results indicate that inv(9) may often cause clinical problems in offspring of the carrier and infertility with unknown mechanisms related to sex. pericentric inversion of chromosome 9 ; prenatal diagnosis ; infertility ; repeated abortion ; sexual difference Pericentric inversion is one type of chromosomal rearrangement, the detection of which has been greatly facilitated by the development of banding techniques. The site of such inversion of chromosome 9 (inv (9)) is now well known to be heterochromatin.In comparison with pericentric inversion of other chromosomes, pericentric inversion of chromosome 9, the incidence of which has been reported to be about 1% in the general population (Ferguson-Smith 1974;Buckton et al. 1976;Mueller and Klinger 1976), has been categorized as a minor chromosomal rearrangement which does not correlate with abnormal phenotypes

The frequencies of chromosome aberrations and development of the bromodeoxyuridine (BrdU)-sensitive fragile site were studied in vitro in Chinese hamster kidney and bone marrow cells and in vivo in Chinese hamster bone marrow cells. Chromosome aberrations in these cell systems were measured in response to different concentrations of BrdU, fluorodeoxyuridine, or both. The fragile site was found in both homologues of chromosome 1 at 1q22. Treatment with BrdU in vitro but not in vivo produces significant chromosome aberrations. About 50% of chromosome aberrations found after treatment in vitro were at the BrdU-sensitive fragile site compared with 12.5% after treatment in vivo. These results show that BrdU is much more potent in vitro than in vivo in inducing both chromosome aberrations and the expression of the BrdU-sensitive site.

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