Mating type proteins are sequence specific DNA-binding proteins that act as master switches in yeast differentiation by controlling gene expression in a cell type-specific fashion. Transcriptional corepressor that, in a/alpha diploid cells, binds cooperatively with the ALPHA2 protein to a 21-bp DNA sequence termed the haploid-specific gene (hsg) operator, to repress transcription of haploid-specific genes and of MATALPHA1. Binds DNA with a high specificity as a heterodimer of A1 and ALPHA2. Only present in a-cells and in a/alpha diploid cells. Was initially thought to be alternatively spliced. This gene is functional with 1, 2 or no introns, but the functional protein is produced only when both introns are spliced from the mRNA. There are three genetic loci for mating type genes in S.cerevisiae. MAT is the expression locus that determines the mating type of the cell, whereas HML (containing HMLALPHA1 and HMLALPHA2) and HMR (containing HMRA1 and HMRA2) represent silenced repositories of mating type information. The mating type is determined by the MAT locus, which contains either a copy of HML or of HMR. Diploid cells are usually heterozygous for the MAT locus. Belongs to the MATA1 family.