Cells can switch their mating type by recombination as we now describe.In addition to the a or a genes present at the MAT locus in each cell, there is an additional copy of both the a and a genes present (but not expressed) elsewhere in the genome.These additional silent copies are found at loci called HMR and HML (Fig. 11-25).These HMR and HML loci are therefore known as silent cassettes. Their function is to provide a “storehouse” of genetic information that can be used to switch a cell’s mating type. This switch requires the transfer of genetic information from the HM sites to the MAT locus via homologous recombination.