Downregulated CDK4 sensitizes breast cancer cells to radiation. Cells stably expressing pLKO.1, shCDK2 or shCDK4 were irradiated at single doses of 0, 1, 2, 4, 6, or 8 Gy γ-rays. After an average of 8 days, cells were fixed and stained with 1% crystal violet; colonies were counted manually. (A) The figure shows colony formation in MCF10A and ER-PR-HER2- cells at 0 and 6 Gy. (B) Survival curves of various cells transfected with pLKO.1, shCDK2 or shCDK4. ER-PR-Her2+cells were only transfected with pLKO.1 or shCDK4. The y axis represents percentage colony formation relative to unirradiated cells of the respective groups; the x axis represents the various radiation doses. The IC50 and statistical significance are presented in Table 2.