FGFR1 and hematopoietic and lymphoid cell neoplasm: Based on these criteria, hematopoietic neoplasms producing eosinophilia are initially classified according to the presence or absence of certain gene variants, including abnormalities (mutant forms) of PDGFRA, PDGFRB, or FGFR1, or the PCM1-JAK2 fusion gene.