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At 30 dpp as at 18.5 dpc, the levels of Sox9 appeared similar in testes of CD1 and B6 strains. In contrast to fetal testes, B6.Ytir gonads that developed as testes or bilateral ovaries did not show significant differences in Sox9 transcript levels compared with those in B6 testes. However, B6.Ytir ovaries of hermaphrodite B6.Ytir mice showed significantly lower levels than B6.Ytir females with bilateral ovaries (Fig. 6B). Finally, at 60 dpp the levels of Sox9 were not significantly different between B6 and B6.Ytir testes. In contrast, in B6.Ytir ovaries, Sox9 transcripts were no longer detected (Fig. 6C). Whole Mount In Situ Hybridization In situ hybridization with a Sox9 probe performed in whole mount samples confirmed the absence of Sox9 transcripts in B6 XX ovaries and their presence in all gonads with genotype B6.Ytir, regardless of whether they developed into testes or ovaries. The extent and distribution of tissue expressing Sox9 within the gonads varied both between contralateral gonads and between gonads of individuals of the same B6.Ytir litter (Fig. 7). At 13.5 dpc, Sox9-expressing cells were not detected in any B6 XX gonads (Fig. 7A). In contrast, Sox9 transcripts were evident in the middle region of all B6.Ytir gonads. Although in some gonads Sox9-positive tissue extends toward the cranial and caudal regions, in others only the middle region was Sox9-positive (Fig. 7, B and C). At 30 dpp, B6 XX ovaries remained Sox9-negative. In contrast, in B6 and B6.Ytir testis Sox9-positive tissue was strongly detected in the seminiferous tubules, whereas in B6.Ytir ovary clusters of randomly distributed Sox9-positive tissue were evident (Fig. 7D). FIG. 7. Detection of Sox9 using whole mount in situ hybridization in fetal and postnatal B6.Ytir and B6 normal gonads. A-C) Embryos at 13.5 dpc. A) A negative reaction in both gonads (g) of a female B6 embryo. B) This B6.Ytir embryo shows that Sox9-positive tissue is more abundant on the gonad of the left side than on the gonad at the right. C) Here the B6.Ytir gonad at the left side shows a narrower distribution of Sox9-positive tissue than the right gonad. Notice Sox9-positive tissue in the tip of the ribs (arrowheads). D) B6.Ytir ovary (Yo) with clusters of Sox9-positive tissue is shown in the middle of three gonads. The gonad at the left is a normal B6 ovary (Xo) devoid of Sox9-positive tissue, whereas the gonad at the right side is a B6 testis (Yt) with strong reaction for Sox9 in the seminiferous tubules. The three gonads shown are from mice at 30 dpp. Bar = 500 ^m (A-C) and 2 mm (D).