The term hypereosinophilia (HE) refers to persistent and marked blood eosinophilia which is documented on two occasions, with or without an additional tissue eosinophilia. The persistence of HE without organ involvements is called hypereosinophilia of undetermined significance (HEUS). Hypereosinophilic syndrome (HES) is defined by blood and/or tissue HE with HE-related organ damage [1]. About 10% of patients with HES is detected with FIP1L1-PDGFRA (F/P) fusion which remains a therapeutic target of tyrosine kinase inhibitor [2].
