The A alpha mating locus of the woodrotting fungus Schizophyllum commune encodes two multiallelic genes, Y and Z, which regulate the A-pathway of development. The Y alleles contain a homeobox, suggesting that the Y proteins may be DNA-binding regulatory proteins. During mating, development is induced when Y from one mating partner interacts with Z from the other mating partner; self combinations of Y and Z are inactive. Two-hybrid analyses indicate that nonself combinations of Y and Z form heteromultimers and self combinations do not. To understand Y-Z binding and self-nonself recognition further we used mutagenesis and chimeras to identify regions in one allele of Z(Z5) that are involved in these processes. Here we report the results, which broadly define regions in Z5 that are essential for activity, Y-Z binding and Z5 allelic specificity.