ABL1 and polycythemia: More than five decades later, the revised 2008 World Health Organization classification [2] of MPN encompasses eight entities, among them CML harboring the BCR-ABL1 translocation and polycythemia vera, which both share a common stem cell-derived clonal heritage, but different phenotypes (leukocytosis with myelemia in CML and polycythemia in polycythemia vera).