Underlying stem cell, myeloid, or eosinophil neoplasm classified according to WHO criteriab, and end organ damage attributable to HE. Eosinophils are neoplastic (clonal) cells; in many patients, rearranged/fusion variants of PDGFRA, PDGFRB, FGFR1, JAK2, STAT5, FLT3, ABL1 or other driver genes, are found. This evidence concerns the gene JAK2 and hereditary elliptocytosis.