In a large cohort of MPN patients, TET2 mutations were detected in 16% of PV, 5% of ET, 17% of PMF, 14% of post-PV MF, 14% of post-ET MF and 17% of blast phase MPN; but TET2 mutations are also described in other myeloid malignancies such as myelodisplastic syndromes (MDS), MPN/MDS syndromes and acute myeloid leukemia with variable, although not unequivocally defined, prognostic impact. The gene discussed is TET2; the disease is acquired polycythemia vera.