FGFR1 and hypereosinophilic syndrome: For HES secondary to hematologic malignancies, particularly those with PDGFRA or PDGFRB gene abnormalities, such as imatinib mesylate, a tyrosine kinase inhibitor is highly effective.[15,16] Pemigatinib may benefit patients with FGFR1 alterations, though currently approved only for cholangiocarcinoma.[17] Glucocorticoids remain first-line therapy for HES, though long-term use carries significant adverse effects.[18] Secondary HES requires targeted therapy (e.g., antiparasitics).