The investigator has previously identified a mammalian gene, DMRT1, likely to play a role in sex determination by virtue of its relatedness to both the male abnormal 3 (mab-3) gene of C.elegans and the doublesex (dsx) gene of Drosophila. The investigator originally cloned the mab-3 gene and showed that it plays a role in the well-defined sex development pathway of C. elegans, where it is needed for the formation of male sense organs and to prevent males from making yolk proteins. He also cloned a human relative, DMRT1, which he showed to be testis-specific in adults. The human gene maps to a region of chromosome 9 required for male development. The mouse homologue, Dmrt1, was also cloned and has been shown to be expressed in the embryonic gonad as soon as this tissue begins to form. Mab-3, dsx and DMRT1 all contain one or two DNA binding domains which chelate zinc but are distinct from zinc finger domains. The objective of the proposed work is to test the role of DMRT1/Dmrt1 in human and mouse sex determination. The first aim will test whether sex-reversed people, XY female and XX males, have mutations in the DMRT1 gene. The second aim is to knock out the Dmrt1 gene in mouse and test the consequences on sexual development and on expression of other sex determining genes. The third aim is to investigate the DNA binding and transcriptional regulatory properties of Dmrt1 both in vitro and in cultured cells, to test whether it physically interacts with other sex determining proteins, and to ask what genes it regulates.