The TET-family enzymes TET1, TET2, and TET3 influence DNA methylation by modifying 5-methylcytosine. Somatic loss-of-function mutations in TET2 are frequently observed in myeloid neoplasms. In this issue of Blood, Li et al. demonstrate that ablation of Tet2 alters the homeostasis and function of hematopoietic stem cells (HSCs) and induces leukemia in mice, as also reported by other groups.