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 acts in conjunction with A1 to repress transcription of haploid-specific genes (By similarity). Forms a heterodimer with A1. There are three genetic loci for mating type genes in K.lactis. MAT is the expression locus that determines the mating type of the cell, whereas HML (containing HMLALPHA1, HMLALPHA2 and HMLALPHA3) 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 TALE/M-ATYP homeobox family.