Sterile worker castes in eusocial insects have been attributed to kin selection theory, in which the reproductive success of a relative is favored, even at the expense of one's own survival and reproduction. In honeybee workers, reproduction is prevented by queen and brood pheromones and by worker policing. However, in the Cape honeybee subspecies, laying workers can act as social parasites by invading foreign colonies, reproducing and becoming pseudoqueens. These traits have previously been mapped to chromosome 13, and now Robin Moritz and colleagues show that alternative splicing of the honeybee gemini gene regulates sterility in workers (Proc. Natl. Acad. Sci. USA, published online 6 September 2011; doi:10.1073/pnas.1109343108). Laying and non-laying Cape workers showed different splice patterns at exon 5 and exon 7 of gemini, with undeveloped worker bees producing the full transcript containing exon 5, and laying workers producing a transcript lacking exon 5. The authors knocked down exon 5 in non-Cape worker bees and observed higher reproductive capacity, showing that removing exon 5 function affects worker sterility. Sequencing of the flanking introns in the parasitic Cape honeybees showed a consistent 9–base-pair deletion of an intronic splice enhancer motif. Absence of this motif in Cape honeybees suggests that it may act as a genetic switch sufficient to convert an altruistic worker into a parasite.