EZH2 encodes the histone methyltransferase subunit of the polycomb repressive complex 2 (PRC2) that leads to H3K27me3 and promotes transcriptional repression. EZH2 loss of function mutations (nonsense, frameshift mutations, occasionally occurring as homozygous mutations) may occur throughout the gene and have been reported in less than 10% of patients with acute myeloid leukemia, myelodysplasia, atypical chronic myelogenous leukemia, primary myelofibrosis and up to 12% of patients with chronic myelomonocytic leukemia. EZH2 loss of function mutations may be more frequent (15%) among cases of T cell acute lymphoblastic leukemia. EZH2 mutations have been independently associated with adverse prognosis in MDS and MDS/MPN. Therapeutic targeting of EZH2 is currently under study for some types of lymphoma and solid tumors.