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It has been well established that the expression of Sry in undifferentiated gonads of mouse embryos initiates the events leading to testicular differentiation. In some cases of male to female sex reversal, Sry mutations or differing Y alleles can result in differentiation of an ovary instead of a testis. An example occurs in gonads of the strain C57BL/6J (B6) when its Y chromosome is replaced by the Y chromosome (Ydom) of some varieties of Mus musculus domesticus. Failure of normal testis formation is observed in all B6-Ydom offspring (B6.Ytir from Tirano, Italy, or B6-Ypos from Poschiavo, Switzerland). About half of the B6-Ydom embryos develop bilateral fetal ovotestes and become fertile males after birth, whereas the other half form ovaries and develop as infertile females. It has been proposed that B6-Ydom sex reversal results from an improper interaction between the Ydom testis-determining gene (Sry) and dominant autosomal genes in the genome of the C57BL/6J strain, but the identity of these genes remains unknown. On the other hand, it was found that transcription of Sry in B6-Ytir gonads starts at the same time as in normal XY gonads, but its expression persists longer in B6-Ytir gonads.