MLN-TK are driven by rearrangements/fusion genes with involvement of PDGFRA, PDGFRB, FGFR1, JAK2, ABL1, or FLT3. This definition implicates eosinophilia as a recurrent finding, which therefore serves as the main trigger for initiation of distinct cytogenetic and molecular analyses conferring to the identification of disease-defining underlying TK fusion genes. The gene discussed is JAK2; the disease is Increased total eosinophil count.