Pedigrees of the 5 families with familial JAK2 V617F⁺ MPNs. Arrows indicate the affected patients. Line through the symbol indicates a patient who died. Family members affected by cancer (CA) are marked in gray. MPN subtype (essential thrombocythemia [ET], polycythemia vera [PV], prefibrotic primary myelofibrosis [pre-PMF]), age at MPN onset and progression, and progression to PV or acute myeloid leukemia (AML) are indicated for each patient. The germline gene variant for each family is depicted above each pedigree. In patient 4, the diagnosis was ET, but pre-PMF could not be excluded.