An exonic mutation in TMPRSS2 [64,98], an indel in ACE1 [64], a mucus-oversecreting MUC5B variant [67,69,99], some HLA class 1 alleles [100,101,102] and blood group O (possibly by lowering the risk of cardiovascular complications) [103] may have a protective effect against severe COVID-19. The gene discussed is TMPRSS2; the disease is COVID-19.