Cell Culture. HeLa (CCL-2) and HEK293 (CRL-1573) cells were purchased from ATCC and cultured in DMEM medium (Gibco) supplemented with 10% FBS, 100 µg/mL penicillin/streptomycin and L-glutamine. Cells were grown at 37 °C and 5% CO 2 , and were kept no longer than 30 passages. All cell lines were routinely tested for mycoplasma contamination using MycoAlert kit from Lonza.