Approximately one-third of Tet2-/- and 8% of Tet2+/- mice died within 1 year of age because of the development of myeloid malignancies resembling characteristics of chronic myelomonocytic leukemia, myeloproliferative disorder-like leukemia, and myelodysplastic syndrome. The gene discussed is TET2; the disease is myeloproliferative disorder.