RPS6KB1 and breast carcinoma: Like in breast cancer cells, the activation of mTORC1 by serum was also inhibited in Rps27l−/− MEFs, as reflected by the reduced phosphorylation of S6K1, S6, and 4E-BP1 upon addition of serum after serum starvation (Fig. 3b), whereas the phosphorylation of AKT showed no difference regardless of Rps27l status (Fig. 3b), indicating that Rps27l disruption indeed inactivates mTORC1, but has no effects on mTORC2.