Roridin E inhibits the receptor tyrosine kinases FGFR3, IGF-1R, PDGFRβ, and TrkB (IC50s = 0.4, 0.4, 1.4, and 1 μM, respectively) and induces cytotoxicity in multiple breast cancer cell lines (IC50s = 0.02-0.05 nM). It also inhibits proliferation in a panel of additional cancer cell lines (IC50s = <0.01 μM).