Alternative splicing of Sxl mRNA in male and female fruit flies. (a) The structure of the Sxl gene, showing the exons, introns, promoters, start codons, and stop codons, and its splicing patterns. In females, transcription from promoter P_e results in a characteristic splicing pattern and the production of a burst of active Sxl protein (top left). Later transcription from P_m, in the presence of Sxl, continues to produce active Sxl protein (bottom left). In males, transcription from P_e is repressed (top right). The absence of Sxl leads to a different splicing pattern of P_m transcripts and premature termination of Sxl (bottom right). (b) Sxl protein mediates differential expression of genes further downstream in the developmental pathway. In females, active Sxl protein produced from P_m leads to production of proteins that facilitate female development—first Tra protein, which then, with Tra-2, facilitates splicing of the mRNA for active Dsx protein (left). In males, the absence of Sxl leads to a different splicing pattern of tra transcripts; a different version of Dsx is produced that leads to male development (right).