G12Si-1 is a selective K-Ras (G12S) covalent inhibitor, which can inhibit oncogenic signaling of K-Ras (G12S). G12Si-1 shows good ability to covalently engage recombinant K-Ras (G12S) at the mutant serine residue. G12Si-1 can also affect nucleotide cycling of K-Ras by blocking Sos-catalyzed exchange and decreasing the rate of EDTA promoted exchange.