The discrepancy in results between our co-expression system and authentic infection might result from differences in the compartmentalization of the protease and S. During transient co-expression, TMPRSS2 and S are both transported to the plasma membrane via the secretory pathway and TMPRSS2 probably cleaves the S1/S2 site during trafficking through the trans-Golgi network, in a manner similar to that established for IAV HA [77]. This evidence concerns the gene TMPRSS2 and infection.