NTera-2, 3T3, and 293T cells were obtained from the American Type Culture Collection (Rockville, MD, USA). Cells were cultured in Dulbecco's modified Eagle medium (DMEM; Invitrogen, Durham, NC, USA) containing 10% fetal bovine serum (FBS; JRH Bioscience, Lenexa, KS, USA), 0.2 mM GlutaMax-1 (Invitrogen), and penicillin/ streptomycin (100 units/mL; Invitrogen). Cells were split every 2 days in a 0.5% trypsin-EDTA solution and maintained until used.