For instance, the differential response in A549 may be because A549 cells are relatively less permissive to SARS-CoV-2 replication owing to lower concentration of ACE2 receptor protein required for viral entry (30), while Calu3 and Vero cell lines exhibit a much higher expression of ACE2. This difference in infection rate leads to differences in the host immune responses as shown previously (15). The gene discussed is ACE2; the disease is infection.