Transcriptional silencing in the yeast Saccharomyces cerevisiae occurs at HML and HMR mating-type loci and telomeres and requires the products of the silent information regulator (SIR) genes. Recent evidence suggests that the silencer- and telomere-binding protein Rap1p initiates silencing by recruiting a complex of Sir proteins to the chromosome, where they act in some way to modify chromatin structure or accessibility. A single allele of the SUM1gene (SUM1-1) which restores silencing at HM loci in strains mutant for any of the four SIR genes was identified a number of years ago. However, conflicting genetic results and the lack of other alleles of SUM1 made it difficult to surmise the wild-type function of SUM1 or the manner in which the SUM1-1 mutation restores silencing in sir mutant strains. Here we report the cloning and characterization of the SUM1 gene and the SUM1-1 mutant allele. Our results indicate that SUM1-1 is an unusual altered-function mutation that can bypass the need for SIR function in HM silencing and increase repression at telomeres. A sum1 deletion mutation has only minor effects on silencing in SIR strains and does not restore silencing in sir mutants. In addition to its effect on transcriptional silencing, the SUM1-1 mutation (but not a sum1 deletion) increases the rate of chromosome loss and cell death. We suggest several speculative models for the action of SUM1-1 in silencing based on these and other data.