A selective inhibitor of the p70 ribosomal S6 kinase (S6K) (1). Blocks the phosphorylation of S6 at concentrations as low as 0.1-0.3 uM with no activity at GSK3beta and Erk1/2 at 10 uM (2). Useful tool to dissect insulin receptor signaling pathways and was employed to show that the action of mTORC1 on SREB P-1c expression is not mediated by S6K (2). Cell permeable.