We then reviewed the epidemiologic and clinical evidence that AAT deficiency increases risk of COVID-19 infection and of more severe disease, and the experimental evidence that AAT inhibits cell surface transmembrane protease 2 (TMPRSS2) — a host serine protease required for SARS-CoV-2 entry into cells — and that this inhibition may be augmented by heparin. The gene discussed is TMPRSS2; the disease is hyperinsulinemic hypoglycemia, familial, 4.