RPS4X and infection: Additional infection experiments with CHO-K1 cells stably expressing CAR on the cell surface (Fig. 2D, CHO-CAR) revealed a markedly increased virus concentration 6 h after infection for all virus strains (Fig. 2F), illustrating that both the parental viruses and the sCAR-Fc-resistant variants use CAR as a cellular receptor, independent of the efficiency of their neutralization by sCAR-Fc.