Most developmental processes show deep conservation across great phylogenetic distances. In contrast, the signal that triggers the primary genetic switch between the sexes has evolved with remarkable rapidity—entirely lacking the “respectable antiquity” (1) seen in other comparable systems. Coupled with the repeat-rich structure of Y chromosomes, this has made the identification of genetically dominant “M” male-determining factors especially challenging. On page 67 of this issue, Krzywinska et al. (2) compared gene transcript sequences from male and female embryos of the malaria mosquito Anopheles gambiae and identified an early-expressed gene on the Y chromosome, designated Yob. Crucially, they show that it controls sex-specific splicing of dsx (double-sex), the conserved binary switch between male and female development (3), fulfilling the criteria for M. Yob partly overlaps, and probably is a better-annotated version of, a previously identified gene called YG2 (4), recently shown to be conserved across the An. gambiae species complex (5).