How do the two sexes come about? The answer to this question is more complex than one would expect. Though sexual reproduction invariably depends on the presence of male and female individuals, the genetic basis of sex determination varies strongly between species. This diversity is particularly evident in houseflies. This insect order provides an excellent model to study how different sex-determining mechanisms have evolved. Research groups in Zurich, Groningen and Göttingen now have important new findings to report in the prestigious scientific journal Science. They have identified the gene for maleness in houseflies.