A. Type-I strains are characterised by a single protein-coding gene, matA (coloured blue, marked above with ‘A’; dictyBase ID DDB_G0289165), which is homologous to matB (also coloured blue, marked ‘B’), one of the three genes present in the type-II version of the locus. The two other genes making up the type-II locus, matC (yellow, marked ‘C’) and matD (green, marked ‘D’), are homologous to the two genes that occupy the type-III version, matS and matT (coloured yellow and green according to homology, marked S and T; gene nomenclature is treated further in the supplementary discussion). The locus lies on chromosome 5 between the genes DDB_G0289171 and DDB_G0289163, which do not vary according to mating-type and are shown here coloured grey. B. Mating compatibility requires the presence of a matA-class gene (blue triangles) and a matS-class gene(yellow circles) in the two gametes. Type-II cells contain a gene of each class, allowing mating with both types -I and -III. The nature of the interactions between genes remains unknown, as does the molecular explanation of how the matB and matC pair are incompatible.