This family consists of several Ustilago mating-type proteins. The b locus of the phytopathogenic fungus Ustilago maydis encodes a multiallelic recognition function that controls the ability of the fungus to form a dikaryon and complete the sexual stage of the life cycle. The b locus has at least 25 alleles and any combination of two different alleles, brought together by mating between haploid cells, allows the fungus to cause disease and undergo sexual development within the plant [1].

1: The b mating-type locus of Ustilago maydis contains variable and constant regions. Kronstad JW, Leong SA; Genes Dev 1990;4:1384-1395. PMID:2227416