Little is known about the genes that regulate telomere length diversity between mammalian species. A candidate gene locus was previously mapped to a region on distal mouse Chr 2q. Within this region, we identified a gene similar to the dog-1 DNA helicase-like gene in C. elegans. We cloned this Regulator of telomere length (Rtel) gene and inactivated its expression in mice. Rtel -/- mice died between days 10 and 11.5 of gestation with defects in the nervous system, heart, vasculature, and extraembryonic tissues. Rtel -/- embryonic stem cells showed telomere loss and displayed many chromosome breaks and fusions upon differentiation in vitro. Crosses of Rtel+/- mice with Mus spretus showed that Rtel from the Mus musculus parent is required for telomere elongation of M. spretus chromosomes in F1 cells. We conclude that Rtel is an essential gene that regulates telomere length and prevents genetic instability.