CEL-NOS constitutes a rare entity within the WHO 2008 classification [5] defined by unexplained eosinophilia greater than 1.5 × 109/L either with evidence of clonal eosinophilia via abnormal cytogenetics (excluding BCR-ABL, PDGFRA, PDGFRB, or FGFR1 rearrangements) or in the presence of greater than 2% or 5% blasts (but less than 20%) in the peripheral blood or bone marrow, respectively. The gene discussed is PDGFRB; the disease is Chronic Eosinophilic Leukemia, Not Otherwise Specified.