Genotypes, interactions and outputs of the mating-type loci of Candida albicans and Saccharomyces cerevisiae [5,15]. (a) The C. albicans MTL locus. The α version of the locus encodes the α1 protein, which activates the expression of α-specific genes; the a version of the locus encodes the a2 protein, which activates the expression of a-specific genes. In a/α cells, where both versions of the locus are present, the α2 and a1 proteins act together as a repressor of the phenotypic switch from white to opaque, and of 'haploid-specific' genes. (b) The S. cerevisiae MAT locus. As in C. albicans, the α1 protein activates the expression of α-specific genes, and α2 and a1 act together in diploid cells as a repressor of MATα1 and haploid-specific genes. The a-specific genes are, however, constitutively expressed in the absence of the α2 repressor. The dotted lines highlight the replacement of a positive branch in C. albicans with a negative one in S. cerevisiae.