Mating-type switching is controlled by recombination of DNA cassettes

S. cerevisiae chromosome III encodes three mating-type loci, but only the genes at the MAT locus are expressed. HML encodes a silent cassette of the α genes, and HMR encodes a silent cassette of the a genes. Copying of a silent cassette and insertion through recombination at the MAT locus switches mating type.