PROS1 and infection: As determined by a pseudovirus-based single-cycle infection assay and a dual-split protein (DSP)-based cell–cell fusion assay, a number of mutations resulted in significantly reduced infectivity and cell fusion capacity relative to the wild-type (WT) S protein, while two mutants (K1205A and Q1208A) enhanced the pseudovirus entry but not the cell–cell fusion (Figure 2).