LJI308 is potent pan-RSK inhibitor that targets RSK N-terminal kinase domain (NTKD) ATP-binding site (IC50/[ATP] = 6 nM/5 µM/RSK1, 4 nM/20 µM/RSK2, 13 nM/10 µM/RSK3), exhibiting similary binding affinity toward RSK1-4, but much reduced inhibitory potency against S6K1 (IC50 = 0.8 µM), MEK4 (IC50 >10 µM), and HIP kinase 1 (IC50 >1 µM). LJI308 effectivelfy reduces cellular Y-box binding protein-1 (YB1) Ser102 (EC50 = 0.2-0.3 µM; MDA-MB-231 and H358 cells), but not ribosomal S6 protein (S6RP) S235/236, phosphorylation level and selectively inhibits the growth of YB1-overpressing HTRY-LT triple-negative breast cancer (TNBC) cells, but not the parental non-tumorigenic human mammary epithelial cell (HMEC) line HTRZ (% inhibition = 6.8%/HTRZ, 88%/HTRY-LT1, 66%/HTRY-LT2 in 8 days; 5 µM at 0 and 96 hr).