Among the array of agents being studied against COVID-19, alpha-1-antitrypsin (AAT)—a member of a superfamily of serine protease inhibitors (serpin) and the most abundant serpin in circulation1—has been shown to inhibit Transmembrane Protease, Serine 2 (TMPRSS2), a cell surface serine protease that is required to process the spike protein of SARS-CoV-2 to allow the virus to gain intracellular entry2–8. The gene discussed is TMPRSS2; the disease is COVID-19.