FGFR1 and hypereosinophilic syndrome: Findings suggestive of specific HES subtypes include rearrangement or mutation of the genes PDGFRA, PDGFRB, FGFR1, or JAK2, or evidence of clonal eosinophilia (M-HES subtype) and clonal T-cell population that produces eosinophil-promoting cytokines (L-HES subtype).