In 1921 it was discovered that the sexual fate of Drosophila is determined by the ratio of X chromosomes to sets of autosomes. Only recently has it been found that the X chromosome to autosome ( X:A ) ratio is communicated in part by the dose of sisterless-b (sis-b), an X-linked genetic element located within the achaete-scute complex of genes involved in neurogenesis. In this report, the molecular nature of the primary sex determination signal and its relation to these proneural genes was determined by analysis of sis-b+ germline transformants. The sis-b+ function is confered by protein T4, a member of the helix-loop-helix family of transcription factors. Although T4 is shared by sis-b and scute-alpha, the regulatory regions of sis-b, which control T4 expression in sex determination, are both separable from and simpler than those of scute-alpha, which control T4 expression in neurogenesis. Dose-sensitive cooperative interactions in the assembly or binding of sis-dependent transcription factors may directly determine the activity of the female-specific promoter of Sex-lethal, the master regulator of sexual development. In this model there is no need to invoke the existence of analogous autosomal negative regulators of Sex-lethal.