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The aim of the present study was to correlate the timing and distribution of pre-Sertoli cells that express Sox9 by immunofluorescence with the profile levels of Sox9 transcripts in fetal and postnatal B6-Ytir gonads. It was found that pre-Sertoli cells appear in all undifferentiated B6-Ytir gonads as in wild-type B6 gonads at 11.5-12.5 dpc. From 13.5 dpc onward, however, pre-Sertoli cells expressing the Sox9 protein were absent in about 50% of the gonads. This corresponds to the number of B6-Ytir gonads that develop as ovaries and in the cranial and caudal regions in fetal ovotestis. In contrast, reverse transcriptase polymerase chain reaction (RT-PCR) revealed the presence of Sox9 transcripts in all fetal and postnatal B6-Ytir gonads, although levels were lower in B6-Ytir ovaries. Thus it is tempting to speculate that abnormal development of B6.Ytir gonads may be due to partial failure of Sox9 function as a consequence of reduced dosage in pre-Sertoli cells, which leads to ovotestis or the development of follicular cells in B6.Ytir ovaries. In turn, the reduced dosage could be a consequence of the reduced ability of Sry to activate Sox9 transcription. Furthermore, the abnormal presence of Sox9 transcripts in B6-Ytir ovaries found here may cause infertility in B6-Ytir females.