PDGFRA and hypereosinophilic syndrome: Patients with HES are further sub-divided into 3 groups according to mechanisms underlying eosinophil expansion: (1) neoplastic HES (HESN), with a demonstrable cytogenetic/karyotypic perturbation resulting in clonal proliferation of eosinophils (most commonly F1P1L1/PDGFRA+ chronic eosinophilic leukemia) or increased blasts, (2) reactive HES (HESR), where increased/deregulated production of eosinophilopoietic factors drives polyclonal eosinophil expansion, and (3) idiopathic HES (HESI) when the mechanism driving eosinophilia remains elusive.