and HMα, which would allow an a/a diploid to sporulate and would cause haploid segregants inheriting the mutant allele to behave as a/α diploids despite being haploid. The authors named the mutation MAR for its apparent role in mating type regulation, and were able to map the mutation to chromosome IV, and determined that it was located 27.3 cM from a commonly used trp1 marker. A few months later, Jasper Rine and Ira Herskowitz published a different screen for genes that affect the ability of yeast to mate, and ultimate discovered the gene family that they called SIR, a name that remains in the modern parlance. Unlike the Klar et al. screen that identified a mutant by its inability to mate, Rine & Herskowitz took a more directed approach towards discovering factors responsible for mating type silencing. Specifically, Rine & Herskowitz reasoned that a haploid yeast cell with a recessive mutation in matα1 could be complemented if the silent copy of MATα were de-repressed. Starting in a ho matα1 haploid strain, Rine & Herskowitz screened mutants arising from mutagenesis and identified five mutants that restored a MATα phenotype in matα cells, but were not linked to the MAT locus and did not cause a gene conversion between the HMα locus and matα. These mutants, they reasoned, were specifically defective in silencing the cryptic mating type genes. Eventually, all of the mutants resulting from the original Hopper & Hall screen as well as the later Rine & Herskowitz screen and the Klar et al. screen were characterized and mapped, and it was shown that the causative genes were the same. In fact, the genes that are now referred to as SIR1-4 have at one time been referred to as MAR, CMT or STE according to the screen that identified the mutants. Although Klar, Hartwell and Hopper identified mutations in SIR genes and applied other names to the genes before Rine performed his screen, the SIR name was eventually adopted because Rine eventually identified the most complete set of functionally related genes (SIR1-4), and because the work by Rine and Herskowitz most accurately described the function of the SIR family genes. Later it would be shown that in yeast and in higher organisms, SIR proteins are important for transcriptional regulation of many chromatin domains. Molecular mechanism In budding yeast, SIR proteins are found at the silent mating type loci, telomeres, and at