The yeast Sir2p protein has an essential role in maintaining telomeric and mating type genes in their transcriptionally inactive state. Mammalian cells have a very large proportion of their genome inactive and also contain seven genes that have regions of homology with the yeast sir2 gene. One of these mammalian genes, sir2α, is the presumptive mammalian homologue of the yeast sir2 gene. We set out to determine if sir2α plays a role in mammalian gene silencing by creating a strain of mice carrying a null allele of sir2α. Animals carrying two null alleles of sir2α were smaller than normal at birth, and most died during the early postnatal period. In an outbred background, the sir2α null animals often survived to adulthood, but both sexes were sterile. We found no evidence for failure of gene silencing in sir2α null animals, suggesting that either SIR2α has a different role in mammals than it does in Saccharomyces cerevisiae or that its role in gene silencing in confined to a small subset of mammalian genes. The phenotype of the sir2α null animals suggests that the SIR2α protein is essential for normal embryogenesis and for normal reproduction in both sexes.