Hormone involved in the regulation of erythrocyte proliferation and differentiation and the maintenance of a physiological level of circulating erythrocyte mass. Binds to EPOR leading to EPOR dimerization and JAK2 activation thereby activating specific downstream effectors, including STAT1 and STAT3. Produced by kidney or liver of adult mammals and by liver of fetal or neonatal mammals. Belongs to the EPO/TPO family.