Abstract The SH2-JH2 linker domain of JAK2 has been implicated in the negative regulation of JAK2 activity. In 2 patients with myeloproliferative neoplasms (MPNs), we identified and characterized the novel JAK2 mutation S523L, which occurs in a key residue in the linker region. In 1 case, acquisition of JAK2S523L was associated with thrombocytosis and bone marrow megakaryocytic hyperplasia, and there were no other somatic alterations in this patient. The second patient with JAK2S523Lmutation presented with increased hematocrit and had concurrent mutations in RUNX1 and BCORL1. Consistent with the genetic and clinical data, expression of JAK2S523L causes interleukin-3–independent growth in Ba/F3 cells transduced with the erythropoietin receptor by constitutively active Jak2/Stat5 signaling.