Mammalian male sex determination is controlled by a complex hierarchy of gene regulatory proteins and hormones, which promote male gonadal development and regression of the female primordia. At the core of this pathway lies the SRY protein, the master developmental switch for testicular differentiation and hence, the male sex. The three-dimensional structure of the SRY-DNA complex suggests a model of developmental gene regulation in which proteins that alter DNA structure and promote the assembly of higher-order nucleoprotein complexes play an essential role in the timing of cell specialization events.