Fig. 7. How SXL promotes exon-skipping. In females, the SXL protein (pink ovals) binds to several sites within the introns located both upstream and downstream of the male-exon (blue rectangle), and associates with both U2AF and the U1 snRNP to block the 3' and 5' splice sites from being used. Formation of this dead-end complex guarantees that the male-exon will be skipped, and that exon 2 is spliced to exon 4. In males, where there is no SXL protein, U2AF and the U1 snRNP are free to assemble into an active spliceosome and exon 3 is included in the mature transcript. AG indicates the location of the two male exon 3' splice sites.