HML and HMR loci are
Yeast possesses three mating type related loci on chromosome III but only MAT is transcriptionally active. HML alpha and HMR a loci are embedded in telomere proximal heterochromatin enriched for Sir2–Sir4 deacetylase complex that removes histone acetylation and establishes hypoacetylated nucleosomes silencing transcription. Rap1 and Abf1 recruit Sir proteins spreading silent chromatin across silencers E and I flanking cassettes. This silencing prevents co-expression of both mating programs simultaneously avoiding sterile phenotype, while allowing them to serve as unexpressed templates for recombination-mediated switching and gene conversion restoring opposite mating type at MAT.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Silent HML HMR Loci and Heterochromatin Silencing