RPS6KB1 (S6K1) contains a TOS motif. mTORC1 requires an intact TOS motif to bind and phosphorylate S6K1 (Ali & Sabatini 2005). Two isoforms of RPS6KB1 are produced by alternative translation, p85 S6K1 (RPS6KB1-1) of 525 amino acids, and p70 S6K1 (RPS6KB1-2) of 502 amino acids that lacks 23 N-terminal amino acids present in p85 S6K1 (Rosner and Hengstschläger 2011). Both isoforms are phosphorylated by the mTORC1 complex, on Thr412 in p85 S6K1 and on the corresponding Thr389 in p70 S6K1 (Rosner and Hengstschläger 2011). p70 S6K1 is more abundant in cells than p85 S6K1 (Rosner and Hengstschläger 2011). Alternative translation is thought to regulate localization of S6K1 (Rosner and Hengstschläger 2011).