The SAD mutation, an extra mating type cassette, has been shown to arise from an unequal mitotic crossover between the MAT and HMR loci, resulting in the formation of a hybrid cassette and a duplication of the MAT-HMR interval. The SAD cassette contains the "a" information and left-hand flanking regions from the parental HMRa cassette and the right-hand flanking sequences of the parental MAT cassette. This arrangement of flanking sequences causes a leaky but reproducible mating phenotype correlated with a low-level expression of the cassette as measured by RNA blotting. This weak expression is attributed to the loss of one flanking control site normally present at the silent HM storage loci.