Genetic model of sex determination in humans. The formation of the undifferentiated/bipotential gonad is controlled by several genes acting simultaneously, such as WT1, SF1 and Lim1. Primary sex determination is based on the presence of the Y chromosome and its main sex-determining gene, SRY. In this case, SOX9, FtzF1/SF1 and AMH expression divert the gonad and the reproductive tract towards the male phenotype. This differentiation process is regulated by several other genes, including DAX1, GATA4, FOXL2 and, possibly, DMRT1 and 2 (not shown in the figure). In females, SRY absence allows gonadal development towards a female phenotype, mediated by genes such as DAX1, Wnt4 and SF1, resulting in aromatase upregulation. The exact role of stra8 (not shown) in this process remains to be clarified.