Chronic eosinophilic leukemia not otherwise specified (CEL-NOS) is characterized by an AEC exceeding 1,500/μL, blast counts below 20%, increased blasts (>2% in blood or >5% in marrow), and demonstration of clonal cytogenetic or molecular abnormalities in the absence of Philadelphia-positive or negative MPN/MDS overlap and absence of PDGFRA/PDGFRB/FGFR1 rearrangements. This evidence concerns the gene FGFR1 and Chronic Eosinophilic Leukemia, Not Otherwise Specified.