In particular, alterations in the TET2 gene, a putative tumor suppressor gene located at chromosomal region 4q24, have been identified in 7–13% of MPN patients, in 19–26% myelodysplastic syndromes (MDSs), in 12–24% of acute myeloid leukemia (AML), in 20–40% of chronic myelomonocytic leukemia (CMML), and in 29% of systemic mastocytosis [2–9]. The gene discussed is TET2; the disease is chronic myelomonocytic leukemia.