To the Editor: Erythrocytosis occurring in association with a subnormal serum erythropoietin level is indicative of underlying erythropoietin-independent erythropoiesis, in patients with polycythemia vera, or erythropoietin-hypersensitive erythropoiesis, in patients with congenital erythrocytosis related to an erythropoietin-receptor mutation.¹ Activating Janus kinase 2 (JAK2) mutations are found in approximately 99% of patients with polycythemia vera.² It is currently assumed that other mutations relevant to JAK signaling contribute to the pathogenesis of JAK2 mutation–negative polycythemia vera or “idiopathic erythrocytosis” in patients with a subnormal serum erythropoietin level.

The lymphocyte-specific adaptor protein (LNK) is a plasma membrane–bound adaptor protein whose function includes . . .