Sox9 is an Sry-box-containing gene that encodes a transcriptional activator. During mouse gonadogenesis, Sox9 is detected in the male gonad at 11.5 days postcoitus (dpc). At 12.5 dpc, testicular cords form, morphologically distinguishing the male gonad from the ovary. From this stage onwards, Sox9 expression is restricted to the Sertoli cell lineage and persists in the adult. Humans with heterozygous mutations in SOX9 develop a skeletal syndrome known as campomelic dysplasia. Furthermore, most XY SOX9 heterozygotes show variable male-to-female sex reversal, implicating SOX9 in testis development. Sox9 heterozygous knockout mice die at birth with a syndrome similar to that of human campomelic dysplasia. In contrast to humans, XY Sox9+/- mice form normal appearing testes. Germ-line knockout of Sox9 using a conditional null allele provides a tool for generating Sox9-/- mice by simple genetic crosses. However, Sox9-/- mice die soon after 11.5 dpc because of cardiovascular defects. In vitro culture of the urogenital ridges of XY Sox9-/- results in gonads lacking testicular cords and Sertoli cell marker expression, but with the expression of ovarian-specific markers. Therefore, Sox9 is essential for diverting an intrinsically ovarian program of organogenesis toward testis formation.