Sex determination pathways vary significantly between species and even within a single species. In fruit flies, the pathway is relatively well understood; splicing appears to play a key role, the RNA-binding proteins Sex-lethal (SXL) and TRA-TRA2 being critical components of the mechanism. Less is known about mammalian sex determination, but a common view is that it depends primarily on transcriptional regulation rather than splicing. In a Hypothesis on p. 441, Paolo Sassone-Corsi and co-workers argue that splicing in fact plays an important part. They focus on two proteins, the Wilms' tumour protein (WT1) and the Sox-family protein SRY, both of which are required for testis differentiation. WT1 isoforms associate with spliceosomes and interact with splicing factors such as WTAP, the homologue of a protein required for female-specific sxl/tra splicing in flies. SRY might function as a transcription factor; however, recent work indicates that it participates in pre-mRNA splicing both in vitro and in vivo. Sassone-Corsi and co-workers suggest that WT1 and SRY play roles similar to those of SXL and TRA in Drosophilaand thus that the role of splicing in sex determination is more conserved across phyla than previously believed.