DPP9 and COVID-19: Critical illness in COVID-19 can be traced to several genes found to function in at least two distinct biological pathways: 1) innate antiviral defenses, which are vital early in disease (IFNAR2 and OAS), and 2) host-driven inflammatory lung injury, which is a crucial mediator of life-threatening COVID-19 (DPP9, TYK2, and CCR2) (2).