Finally, to avoid the diagnostic pitfall of SM, FISH testing for PDGFRA, PDFGRB, and FGFR1 gene rearrangements is recommended to exclude myeloid/lymphoid neoplasms with eosinophilia, as it is not uncommon to find aberrant mast cell populations (with expression of CD2 and/or CD25) in these neoplasms with flow cytometry. The gene discussed is FGFR1; the disease is systemic mastocytosis.