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 binds cooperatively with MCM1 to a 31-basepair DNA sequence termed the a-specific gene (asg) operator, to repress the transcription of a-cell-specific genes. Additionally, in a/alpha diploid cells, binds cooperatively with the A1 protein to a 21-basepair DNA sequence termed the haploid-specific gene (hsg) operator, to repress transcription of haploid-specific genes and of MATALPHA1