Regarding hematological forms, the 2016 WHO Classification first approved a semi-molecular classification scheme of disease subtypes, including the following main categories: 1) myeloid/lymphoid neoplasms with eosinophilia (M/LN-Eo) and PDGFRA/B or FGFR1 rearrangement or with PCM1-JAK2; 2) myeloproliferative neoplasm (MPN) subtype: chronic eosinophilic leukemia, not otherwise specified (CEL, NOS); and 3) idiopathic HES (iHES), which is a diagnosis of exclusion [2]. The gene discussed is FGFR1; the disease is Chronic Eosinophilic Leukemia, Not Otherwise Specified.