IFITM1 and viral infectious disease: Because the C-terminal variable regions of IFITM1 and IFITM3 control the inhibition and enhancement of HCoV-OC43 entry (42), respectively, we further compared the effects on virus infection of overexpressing IFITM1-EX2, a mutant IFITM1 protein with its C-terminal domain replaced with the C-terminal domain of IFITM3 (42), and IFITM3-EX2, a mutant IFITM3 protein with its C-terminal domain replaced with the C-terminal domain of IFITM1 (42).