A mutation has been identified that suppresses the mating and sporulation defects of all mutations in the mating-type loci of S. cerevisiae. This suppressor, sir1–1, restores mating ability to matα1 and matα2 mutants and restores sporulation ability to matα2 and mata1 mutants. MATa sir1–1 strains exhibit a polar budding pattern and have reduced sensitivity to α-factor, both properties of a/α diploids. Furthermore, sir1–1 allows MATa/MATa, matα1/matα1, and MATα/MATα strains to sporulate efficiently. All actions of sir1–1 are recessive to SIR1. The ability of sir1–1 to supply all functions necessary for mating and sporulation and its effects in a cells are explained by proposing that sir1–1 allows expression of mating type loci which are ordinarily not expressed. The ability of sir1–1 to suppress the matα1–5 mutation is dependent on the HMa gene, previously identified as required for switching of mating types from a to α. Thus, as predicted by the cassette model, HMa is functionally equivalent to MATα since it supplies functions of MATα. We propose that sir1–1 is defective in a function, Sir ("Silent-information regulator"), whose role may be to regulate expression of HMa and HMα.