A decrease in growth rates has also been reported previously for radioresistant prostate cancer cell lines derived from surviving clones of irradiated tumors (38) and for radioresistant breast cancer cell lines generated through X-ray-conditioning, where the reduction in proliferation was linked to a reduced expression of the MKI67 gene encoding the marker of proliferation Ki67 (19). The gene discussed is MKI67; the disease is prostate cancer.