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It has a tetrapolar mating system with each cell containing two mating-type loci (called A and B) that govern different aspects of the mating process, leading to 4 possible phenotypes after cell fusion. Each locus codes for a mating type sublocus (α or β) and each type is multi-allelic: the A locus has 9 alleles for the α type and an estimated 32 for its β type, and the B locus has 9 alleles each for both its α and β types. When combined this gives an estimated 9 × 32 × 9 × 9 = 23328 potential mating type specificities. This does not mean all different mating types are compatible with one another, because compatibility between haploid individuals exists only when for both the A and the B mating-type locus at least the α or β are different. Strains are thus compatible with ( 1 − 1/ (9 × 32) ) × ( 1 − 1/ (9 × 9) ) ≈ 0.984 = 98.4 % of the population.