RPS6KB1 and breast cancer: Furthermore, we found that CD44high/CD24low MCF7 cells, a breast cancer stem cell line with a radioresistant phenotype24,25 (Supplementary Fig. S2), expressed higher levels of p-S6K1 compared to those of control MCF7 cells (Fig. 2b), suggesting that high expression levels of p-S6K1 are involved in the radio-resistance of breast cancer.