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 ALPHA2 to repress transcription of haploid-specific genes (By similarity). Required for induction of sporulation in diploid cells. Forms a heterodimer with ALPHA2. Only present in A-cells and in A/B diploid cells. Belongs to the MATA1 family.