Fig. 1. Somatic portion of the sex determination hierarchy. In females, Sxl is activated and regulates the splicing of tra pre-mRNA, resulting in the production of TRA. TRA together with TRA2 regulates the female-specific splicing of dsx and fru pre-mRNAs. In males, Sxl is not activated and no functional TRA is produced, resulting in default splicing of dsx and fru pre-mRNAs. dsx produces both female- and male-specific isoforms. fru produces a male-specific isoform.