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After seminiferous cord formation, Sox9 expression is specific to Sertoli cells, and Sox9 is probably one of the first genes induced by Sry in male gonads. The DNA-binding domains of both Sry and Sox9 proteins are highly conserved and can functionally substitute for each other. A mouse line with a transgene inserted upstream of Sox9 showed that the development of XX males is consistent with Sry being an indirect repressor downstream target of Sox9. Recently, female to male sex-reversal induced by ectopically expressing Sox9 in XX gonads indicated that it can substitute for Sry in the events necessary to form a testis and is likely to act downstream. It is assumed that Sry and Sox9 are expressed in pre-Sertoli cells from which an orchestrated series of intercellular signals leads the undifferentiated genital ridge to form a testis. Since it is known that Sry is expressed in B6-Ytir fetal gonads, it seems likely that formation of ovotestis or ovaries in B6-Ytir gonads may be due to a mismatch of intercellular signals before and/or after Sox9 expression in pre-Sertoli cells.