Baby Hamster Kidney (BHK)-21 cells (clone 13) were obtained from the European Cell Culture Collection and maintained at 37uC, 5% CO 2 , in Glasgow Minimum Essential Medium (GMEM) (Sigma) containing 10% foetal bovine serum (FBS) (Autogen Bioclear), 2 mM glutamine, 100 U/ml penicillin, 100 mg/ml streptomycin and 5% tryptose phosphate broth solution (Sigma).