Clonal eosinophilia is seen in the context of a myeloid neoplasm and particularly the World Health Organisation (WHO)-defined entities ‘chronic eosinophilic leukemia, not otherwise specified’ (CEL-NOS) and ‘myeloid and lymphoid neoplasms with rearrangement of PDGFRA, PDGFRB or FGFR1 or with PCM1-JAK2, ETV6-JAK2 or BCR-JAK2’ (MLN-eo). The gene discussed is JAK2; the disease is lymphoid neoplasm.