Cells.

RO6E/J cells were a kind gift from Ingo Jordan (CureVac AG, Germany) and were maintained in Dulbecco’s modified Eagle medium (DMEM)–F-12 plus GlutaMAX (Thermo Fisher) complete medium (10% heat-inactivated [56°C, 30 min] fetal bovine serum [FBS] and 50 mg/liter [final concentration] gentamicin sulfate [Cellgro]) at 37°C under 5% CO₂. EpoNi/22.1 cells were a kind gift from Christian Drosten (The Charité—Universitätsmedizin Berlin) and were maintained identically to RO6E/J cells. HepG2 cells were obtained from the ATCC (HB-8065) and were maintained in DMEM low-glucose (1 g/liter) complete medium (Thermo Fisher) at 37°C under 5% CO₂. HEK 293T/17 cells (referred to below as “293T cells”) were obtained from the ATCC (CRL-11268) and were maintained in DMEM high-glucose (4.5 g/liter) complete medium (Thermo Fisher) at 37°C under 5% CO₂. 769-P cells were acquired from the ATCC (CRL-1933) and were maintained in RPMI 1640 complete medium (Thermo Fisher) at 37°C under 5% CO₂. Dicer knockout NoDice 4-25 cells and the parental cell line (referred to as “293T-P cells” to distinguish them from the 293T cells obtained from the ATCC) were a kind gift from Bryan Cullen (Duke University School of Medicine) and were maintained identically to the 293T cells obtained from the ATCC. BHK-21 C13 (CCL-10) and Vero E6 (CRL-1586) cells were obtained from the ATCC and were maintained identically to HepG2 cells.