FIP1L1 and Chronic Eosinophilic Leukemia, Not Otherwise Specified: The most frequent genetic aberration is the cryptic deletion of 4q12, i.e. del(4)(q12), producing the FIP1-like 1/platelet-derived growth factor receptor alfa (FIP1L1-PDGFRA) fusion transcript, which results in an eosinophilic, myeloproliferative disorder (chronic eosinophilic leukaemia, CEL) [5].