JS-K decreases p38 phosphorylation in breast cancer cells. Protein lysates were obtained from MDA-MB-231, F10, and MCF-7/COX-2 cells treated in the absence or presence of JS-K for 24 hours. Western blot analyses using phospho(Thr180/Tyr182)-p38, p38, phospho(Thr202/Tyr204)-extracellular signal-regulated kinase 1/2 (ERK1/2), ERK1/2, phospho(Thr183/Tyr185)-c-Jun N-terminal kinase (JNK), and JNK antibodies were performed. β-actin was used as a loading control. Images were scanned and quantified. Ratios of phospho-p38 to total p38 levels in JS-K-treated cells were determined and compared with those in untreated cells. Ratios of phospho-ERK1/2 to total ERK1/2 levels in JS-K-treated F10 cells were determined and compared with those in untreated F10 cells.