Purity >98%. LJI308 is a selective and potent inhibitor of the p90 ribosomal S6 kinase (RSK) family with IC50 values ranging from 4 to 13 μM for RSK isoforms 1-3. In a panel of 442 kinases, LJI308 is selective for RSK1, RSK3, and RSK4. It also inhibits S6K1, MEK4, and HIP in the low micromolar range but had no effect on other kinases tested. LJI308 also reduces phosphorylation of Y-box-binding protein (YB1) in MDA-MB-231 cells bearing activating mutations in the MAPK signaling pathway (EC50 = 0.21 μM) and reduces H358 growth in soft agar and colony forming assays.