The yeast Saccharomyces can switch its mating type by a highly choreographed recombination event in which 'a' or 'alpha' sequences at the mating-type (MAT) locus are replaced by opposite mating-type sequences copied from one of two donors, HML and HMR, located near the two ends of the same chromosome III. MAT alpha cells 'know' to choose HML, while MAT alpha cells preferentially recombine with HMR. Donor preference is regulated by a 250 bp recombination enhancer, that controls recombination of the entire left arm of chromosome III. Recent studies have shown how this locus-control region is turned on and off.