The expression of critical inhibitory checkpoint proteins of T cell exhaustion, including PD-1 and Tim-3, and the increase of TNF-α, IL-10 and other cytokines (which actually all may modulate the expression of PD-1 and Tim-3) are likely to mediate T cell lymphopenia in patients with COVID-19 through programmed cell death.73 The gene discussed is PDCD1; the disease is COVID-19.