Clonal hypereosinophilia is hypereosinophilia caused by a pre-malignant or malignant clone of eosinophils that bear mutations in genes for "PDGFRA", "PDGFRB", or "FGFR1" or, alternatively, a chromosome translocation that creates the "PCM1-JAK2" fusion gene. These genes code for dysfunctional protein products capable of enhancing proliferation and/or survival of their parent cells which, in consequence, become an evolving and constantly growing clone of eosinophils. These mutations are recognized by the World Health Association as causing distinct entities differing from idiopathic hypereosinophilia and the idiopathic hypereosinophilic syndrome. Presence of these clones may be associated with tissue injury but in any case suggests specific therapy be directed at reducing the size and suppressing the growth of the eosinophil clone. More recently, mutations in other genes have been described as causing a similar type of clonal hypereosinophilia but have not yet been recognized as entities distinct from idiopathic hypereosinophilia and the idiopathic hyperesoniphilic syndrome. These include gene mutations in "JAK2", "ABL1", and "FLT2" and chromosomal translocations that create the "ETV6-ACSL6" fusion gene.