EPO and neoplasm: It is currently unknown whether EPOR in cancer cells exists as a homodimer or as a heterodimer [with the common-β receptor subunit (CD131)] with a much lower EPO affinity (50, 51), whether EPOR activation is ligand-independent (52), or whether endogenously produced EPO, either by the kidney or by the tumor itself (2, 10, 53), is sufficient to fully activate EPOR in A549 tumors.