The mechanisms for sex differentiation and the genes on the sex chromosomes are varied among different species. For human, SRY is the only testis-determining factor on the Y chromosome and triggers the cascade for male sex-determination. However, even if normal SRY exists, the haploinsufficienty of SOX9 or KTS+ splicing form of WT-1 can cause male-to-female sex reversal. Furthermore, the duplication of the partial region on the X chromosome including DAX-1 gene can also cause male-to-female sex reversal. The sex-determining system seems to be sensitive for the gene dosage or the gene expression level.