In mammals, sex is determined by the presence or absence of a single gene on the Y chromosome, Sry. Sry, a member of the high mobility group family of transcription factors, is required to initiate male-specific pathways and repress female-specific pathways. Expression of Sry in the gonad, beginning at 10.5 days postcoitum, leads to the differentiation of the somatic supporting cell precursors as Sertoli cells. These cells direct the other cells of the gonad into their respective lineages. Currently, no direct targets of Sry are known. A number of cellular pathways initiated by Sry are required for testis development. These include the proliferation of pre-Sertoli cells and commitment to the Sertoli lineage, migration of cells from the adjacent mesonephros, and formation of a male-specific vasculature. Work is underway to identify genes controlling these processes. These genes will then be linked to Sry.