Sir proteins refer to silent information regulators. In yeast, the Sir proteins are necessary to keep the genetic information in the HMR and HML cassettes silent. Their functions facilitate the condensation of chromatin and help lock up HMR and HML in chromatin domains that are inaccessible to transcriptional activators. Mutations in SIR genes are sterile since both a and α information is expressed in the absence of this specific gene silencing.