We used MCF-7 and T-47D (HR-positive and HER2-negative) breast cancer cell lines, and different doses of CDK4/6 inhibitors (ribociclib and palbociclib) and radiotherapy...When compared with control, ribociclib, and RT group, combined ribociclib and RT significantly shrunk the tumor growth of MCF-7 cell xenograft tumors...The synergistic effects of combining RT with different concentrations of ribociclib and palbociclib pretreatment were demonstrated by colonogenic assay....Our results demonstrated that combining CDK4/6 inhibitors enhanced the radiosensitivity in HR-positive and HER2-negative breast cancer cells...