The human hepatic cell lines HepG2 and Huh7 (kindly provided by Dr Thomas Pietschmann, Department of Molecular Virology, University of Heidelberg) as well as HeLa cells were maintained in Dulbecco's high glucose modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum, 2 mM l-glutamine and 100 U/ml penicillin–streptomycin.