The mechanisms of EPO actions are not well understood, but it has been suggested that an active crosstalk with other growth factor receptors is involved, especially those from the estrogen family and HER2.26,27 It has been shown that the AP-1 (FOS and JUN) transcription factor is critical for growth and proliferation of breast cancer cells.28 We therefore analyzed early gene response in MCF-7 cells stimulated with rHuEPO. Here, JUN is linked to breast cancer.