Secondary (reactive) eosinophilia is mainly driven by cytokines, whereas primary (clonal) forms of eosinophilia are mostly caused by tyrosine kinase gene fusions, involving the coding genes for platelet-derived growth factor receptor alpha (PDGFRA), beta (PDGFRB), or fibroblast growth factor receptor 1 (FGFR1). The gene discussed is FGFR1; the disease is Increased total eosinophil count.