(A) Schematic representation of JAK2. Main acquired mutations in PV and CEL leading to amino acid changes are indicated: JAK2^V617F and JAK2^ex12 for PV and JAK2^V617F and JAK2^ex13indel for CEL. (B) JAK2^ex13indel is a gain-of-function mutant that can activate EPOR in erythroblasts and the βc subunit of IL-5R in eosinophil precursors independently of cytokine, leading to downstream activation of signaling pathways that results in both PV and CEL phenotypes. FERM, band 4.1-ezrin-radixin-moesin; P, phosphorylation.