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. Only present in B-cells and in A/B diploid cells. Belongs to the TALE/M-ATYP homeobox family.