ACE2 and infection: DPP4 is less common in cells of the upper respiratory tract of humans [35,88], the location most favorable to onward transmission [89], than in the lower respiratory tract, making infection less likely; whereas angiotensin converting enzyme 2 (ACE2), the receptor used by SARS-CoV-1 and SARS-CoV-2, is common in the upper respiratory tract [90].