COS-1 cells (African green monkey kidney cells) used for transient transfection were cultured in Dulbecco's modified Eagle's medium (DMEM) containing 2-10% heat-inactivated Foetal Calf Serum (FCS, Gibco™), 2 mM glutamine, 50 mg ml -1 penicillin and 50 mg ml -1 streptomycin in a humidified atmosphere of 5% CO2 at 37°C.