EPO and renal carcinoma: Phosphorylations of the EpoR-related signaling molecules were at higher levels even in the absence of exogenous Epo and after serum starvation for 24 hours, and exogenous Epo had little or no further increase of phosphorylation levels, indicating the constitutional activation of Epo/EpoR signaling pathway in primary renal cancer cells, 786-0 and Caki-1 cells.