Cell-permeable. ATP-competitive inhibitor of the four isoforms of ribosomal S6 kinase (p90 RSK) (IC₅₀'s at 100 µM ATP: RSK1 = 31 nM, RSK2 = 24 nM, RSK3 = 18 nM, RSK4 = 15 nM). Does not significantly inhibit 10 other AGC kinases, and over 40 other protein kinases at 100X concentrations.  In addition, BI-D1870 completely prevents PMA-induced TSC2 (a regulator of mTORC1) phosphorylation at Ser¹⁷⁹⁸.