Although factors involved in male sexual differentiation have been well studied, the pathways regulating female sexual differentiation remain incompletely defined. To date, no genes have been identified to play a similar role in ovarian development as was shown for the SRY or SOX9 genes in testicular development. In mice, Wnt4 regulates the development of the female reproductive tract, antagonizes the production of testosterone, and is important for oocyte development. The recent demonstration of heterozygous WNT4 defects in patients with Müllerian agenesis and signs of ovarian hyperandrogenism added WNT4 to the growing list of genes such as SRY, SOX9, WT1, DAX1, and SF-1 contributing to human sexual development. In particular, WNT4 was the first human gene to be identified to direct development of the bipotential gonad towards ovaries. From a more clinical point of view, it seems that the absence of a uterus (and not other Müllerian abnormalities) and the androgen excess are the pathognomonic signs of WNT4 defects, suggesting that WNT4 deficiency might be a clinical entity distinct from the typical Mayer-Rokitansky-Kuster-Hauser syndrome.