Combinatorial control of the yeast mating-type switch. In all S. cerevisiae cells, haploid and diploid, Mcm1 is expressed and is used in combinatorial control. (a) The haploid a cell expresses protein a1, but this protein is used only in diploid cells. Mcm1 alone turns on a-specific genes (aSG). Other haploid-specific genes (hSG) are also expressed. (b) The haploid α cell expresses the α1 activator and α2 repressor; α2 associates with Mcm1 to turn off α-specific genes, and α1 binds Mcm1 to turn on α-specific genes (αSG). Other haploid-specific genes are expressed. (c) Diploid cells express both a1 and α2. Each, in conjunction with Mcm1, represses transcription of a set of genes: a1-Mcm1 represses haploid-specific genes, and α2-Mcm1 represses a-specific genes. Because α1 is not expressed, α-specific genes are also not expressed.