Pedigrees of 2 families with myeloproliferative disorders characterized by intra-family discordance of JAK2 mutation type. Each family member is identified by a pedigree number. Circles represent female family members; squares male family members; filled symbols members with phenotypically expressed myeloproliferative disorder; slashes members who had died. In family 1, 2 brothers (II-1 and II-5) had PV. Subject II-1 presented at the age of 57 with ischemic stroke, and at the age of 63 he was diagnosed with PV (erythrocytosis and thrombocytosis). This patient was later found to be JAK2 (V617F)-positive, and his granulocyte mutant alleles increased from 6% to 20% over time. He died at the age 69 of pulmonary embolism despite low-dose aspirin and cytoreductive treatment. His brother (subject II-5) was found to have erythrocytosis on routine blood counts at the age of 58; a diagnosis of PV was made, and venesection therapy was started. He was found to carry the N542-E543del mutation in circulating granulocytes. In family 7, 2 sisters (II-1 and II-5) had PV. Subject II-5 presented at the age of 59 with erythrocytosis and thrombocytosis. When she was admitted to the Department of Hematology, Pavia, Italy, at the age of 78, she was found to have post-PV myelofibrosis: she had splenomegaly (below the umbilical line), elevated circulating CD34⁺ cell count (244 × 10⁶/L), and 91% granulocyte JAK2 (V617F) mutant alleles. Her sister (subject II-1) presented at the age of 53 with erythrocytosis, was diagnosed with PV, and started venesection therapy. She was found to carry the R541-E543delinsK mutation in circulating granulocytes.