Mating type genes play a vitally important role in basid-iomycetes by controlling the establishment of heterokaryotic mycelia and subsequent formation of fruiting bodies (Brown and Casselton, 2001). Homobasidiomycetous agarics have either bipolar or tetrapolar mating systems. The former consist of a single mating type locus, locus A, which contains two mating type genes, HD1 and HD2