Abstract
The yeast Saccharomyces can switch its mating type by a highly choreographed recombination event in which `a' or `α' 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. MATα cells `know' to choose HML, while MATα 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.