JAK2 and Increased total eosinophil count: The frequency of eosinophilia varied significantly depending on the specific fusion gene involved: it was present in 91% of patients with FIP1L1::PDGFRA and 100% with ETV6::ABL1, but only in 43% of those with PDGFRB, FGFR1, or JAK2 fusion genes [4, 5].