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Furthermore, in the present study Sox9 transcripts were detected by RT-PCR in both ovotestis and ovaries of B6.Ytir embryos at all developmental stages. This finding suggests that failure to up-regulate Sox9 in pre-Sertoli cells may be posttranscriptional of either Sry, Sox9, or both. Assuming that cells immunopositive to Sox9 detected at 11.5 and 12.5 dpc in B6.Ytir genital ridges are pre-Sertoli cells, current results indicate that 13.5 dpc represents a critical stage for the developmental fate of these gonads. In B6.Ytir ovotestis, seminiferous cords comprising Sox9-positive cells gradually develop toward the cranial and caudal regions of the gonad after 14.5 dpc. Although scattered Sox9-positive cells remain among the sex cords in the ovarian regions, the great abundance of pyknotic cells indicates that in most fetal ovotestes the ovarian tissue regresses. Besides lacking clear epithelial cord organization, the ovarian regions of B6.Ytir ovotestis were identified by the onset of meiosis of the germ cells in these areas. Thus current results support previous observations suggesting that postnatal ovotestes in litters of B6.Ytir mice are rare because most or all ovarian tissue regresses before birth. The levels of Sox9 transcripts at 30 dpp are lower in unilateral (ovary and testis in each side) than bilateral B6.Ytir ovaries (two ovaries), but at 60 dpp Sox9 transcripts are undetected in both. At 30 dpp, unilateral ovaries had more growing follicles than bilateral B6.Ytir ovaries. By 60 dpp, most follicles disappeared in all B6. Ytir ovaries (unpublished results). Sox9 transcripts are absent in normal ovaries. Correlating the levels of Sox9 transcripts with the ovarian morphology suggested that the lower levels of Sox9 transcripts observed in unilateral ovaries at 30 dpp may favor follicular development. Considering that infertility in B6.Ytir females occurs at the ovarian level, current results suggest that the abnormal presence of Sox9 transcripts may disturb oocyte development in follicles of B6.Ytir ovaries.