TMPRSS2 and infection: As the spike protein of SARS-CoV-2 induces cell–cell fusion and syncytiation when exogenously expressed in the host cell membrane during infection [21,26,27], we sought to establish a qualitative in vitro cell–cell fusion assay using transfected HEK293T cells to investigate TMPRSS2-independent factors involved in syncytiation (Figure 1A).