The fungus Phycomyces blakesleeanus is perhaps best known as the light-sensing model pioneered by Nobel laureate Max Delbrück. Work reported in this issue may raise its profile as a model in a different area, the development of sex determination. In this, as in other fungi, sex determination is not controlled by a whole chromosome, but by a small region of the genome. A combined genomic, genetic, molecular and bioinformatic approach has been used to identify this sex locus. Two genes are involved, encoding transcription factors that, like the SRY protein involved in sexual differentiation in humans, contain a high mobility group domain. In addition, a set of repetitive elements is found only on the chromosome containing the sex locus, suggesting a general mechanism for the early steps in the evolution of sex determination and sex chromosome structure in eukaryotes. On the cover, a spiralled hypha of a Phycomyces strain in which both sexM and sexP genes are co-expressed to induce the formation of this pseudo-sexual structure.