Cells BHK-21 cells were obtained from the German Cell Culture Collection (DSZM, Braunschweig, Germany) and grown in Earle's minimal essential medium (EMEM) supplemented with 5% fetal bovine serum (FBS). BHK-G43, a transgenic BHK-21 cell clone expressing VSV G protein in a regulated manner, was maintained as described previously [23] . The porcine kidney cell line PK-15 (ATCC, Manassas, VA) was propagated in Dulbecco's modified Eagle medium supplemented with nonessential amino acids, 1 mM Na-pyruvate and 5% horse serum. D-17 canine osteosarcoma cells (ATCC), Calu-3 human lung adenocarcinoma cells (ATCC), and NHDF normal human dermal fibroblasts (Lonza, Cologne, Germany) were maintained in EMEM with 10% FBS. The UMNSAH/DF-1 (DF-1) chicken fibroblast cell line (ATCC) was maintained in Dulbecco's Modified Eagle's Medium and 10% FBS. All cell lines were cultured at 37uC in a humidified atmosphere containing 5% CO 2 , except DF-1 cells which were kept at 39uC. BALB 3T3 fibroblasts (subclone A31) were kindly provided by N. Pringle, University College, London, UK, and maintained in Dulbecco's Modified Eagle's Medium.