This view is based on the high frequency (60%) of these mutations in CMML patients (9, 19, 21, 22), the fact that TET2 mutated clones can be detected in a small fraction of older subjects with clonal, but non-leukemic hematopoiesis (90–93), the competitive advantage of murine and human HSC invalidated for TET2 (48, 85) and the results of single-cell clonal tracking experiments indicating that a TET2 mutation, when present, is often the earliest recurrent genetic event in CMML (84). The gene discussed is TET2; the disease is chronic myelomonocytic leukemia.