ACE2 and lung adenocarcinoma: In this system, HEK293T cells (without endogenous ACE2 expression) transfected with plasmids encoding WT spike readily formed syncytia after coculturing with HEK293T cells expressing human ACE2 (HEK293T-ACE2) and Vero E6-ACE2, as well as colorectal adenocarcinoma Caco-2 and lung adenocarcinoma Calu-3 cells expressing endogenous ACE2 (SI Appendix, Fig. S1A).