LJI308 is a potent pan-ribosomal S6 kinase (RSK) inhibitor, with IC50s of 6 nM, 4 nM, and 13 nM for RSK1, RSK2, and RSK3, respectively. LJI308 inhibits the phosphorylation of RSK (T359/S363) and YB-1 (S102) after irradiation, treatment with EGF, and in cells expressing a KRAS mutation.