Publisher Summary This chapter discusses the properties of Sry gene with respect to the cellular events underlying differentiation of the gonad and sex determination. The process of sex determination in mammals is believed to depend on the pivotal event during embryogenesis that diverts the development of the indifferent gonad along the male or female pathway. With the isolation and characterization of Sry, a gene with a critical role in development is defined. Sry is described as a master regulatory gene in that it has a pivotal role in the sex determination pathway. The precise pattern of expression of Sry suggests that it must be regulated by tissue-specific transcription factors, or a unique combination of factors present within the supporting cell precursor lineage. These factors are clearly not sex-specific. Second, there are likely to be gene products that interact with the Sry protein and contribute to the specificity of its interaction with target genes. Third, the downstream target genes need to be defined. Sry must initiate the expression of some critical genes. Sry appears to act only for a brief period before testis cord formation; therefore, it is not required for any long-term maintenance of gene activity.