A potent, non-cytotoxic, pyrimidinylthiourea that targets Cdc42 GEF ITSN/intersectin and adjacent GTP/GDP binding site and effectively inhibits EGF-stimulated Cdc42 activity (1 h pretreatment; 50 µM) and Cdc42-dependent microspike/filopodia formation in serum-starved Swiss 3T3 fibroblast cultures without affecting RhoA- or Rac1-dependent functions.