At a time when male embryos undergo rapid differentiation, reorganization of the female gonad is subtle, with few obvious changes. To date, a female sex-determining gene has not been identified. Several years ago mice lacking Wnt4 were generated; these mice exhibited a partial female-to-male sex-reversal, but lacked embryonic testis cords (5). Consistent with this result, overexpression of Wnt4 alone in XY gonads does not result in sex-reversal (6,7), suggesting that Wnt4 is not the sole female sex-determining gene and that additional factors are required to effectively disrupt the male pathway and promote female development.