Two 46,XY females with tandem duplications of an X short arm segment were studied by cytogenetic and Southern blot analysis. The results show that the duplicated segment in each case included the Xp21.2-Xp22.2 interval, resulting in a double dose of ZFX on the single active X chromosome. The results from our two cases, in conjunction with those reported by other workers, lead us to conclude that the duplication is the reason for the sex inversion. If ZFY and ZFX are indeed sex-determining gene loci, these findings favour a model of sex determination characterized by antagonistic interaction between these genes.