Stem cells are believed to possess special machinery that allows them to replicate indefinitely without accumulating defects. However, embryonic stem and embryonic germ cells often acquire chromosomal and imprinting abnormalities in culture, making the findings of Kanatsu-Shinohara and colleagues (see p. 4155) somewhat surprising. These authors cultured spermatogonial stem cells, derived from newborn mouse testes, for two years and found them to be remarkably genetically and epigenetically stable over this time. The cells' stability is reflected in their ability to produce normal sperm when transplanted into infertile mice, and by the fact that this sperm can generate normal, fertile offspring when used in IVF experiments. The only detectable change, the authors report, is a shortening of the telomeres of the stem cells, but this occurs so slowly that it should not hamper the future use of these cells as a tool for genetic modification.