Hypereosinophilic syndrome (HES) is a heterogeneous group of rare disorders characterized by marked blood or tissue eosinophilia resulting in a wide variety of clinical manifestations. A number of HES subtypes, or variants, have recently been identified. The best characterized of the clonal molecular defects implicated in myeloproliferative variant HES is the 800-kb interstitial deletion on chromosome 4q12 resulting in the fusion of 2 distinct genes, Fip1-like 1 (FIP1L1) and platelet-derived growth factor receptor-a (PDGFRA), leading to the FIP1L1-PDGFRA fusion kinase. PDGFR is a membrane-bound tyrosine kinase receptor and the fusion PDGFRA kinase has constitutive/unregulated tyrosine kinase activity. The lymphocytic HES variant is associated with T-cell clones producing interleukin-5 (IL-5) and can evolve into lymphoma. While myeloproliferative and lymphocytic HES are well established and permit elimination of the term, idiopathic, to these varieties, most HES patients do not fall into these categories and are classified as complex.
