EPO and polycythemia: Erythrocytosis can be classified into three disease entities: (1) Congenital/hereditary erythrocytosis caused by an inheritance of genetic traits promoting erythropoietin (EPO) signaling or production; (2) polycythemia vera (PV), a hematological malignancy caused by the acquisition of somatic mutations in hematopoietic stem/progenitor cells promoting erythropoiesis; and (3) non‐neoplastic erythrocytosis (NNE), which includes reactive erythrocytosis, relative erythrocytosis, and idiopathic erythrocytosis.1