In humans, abundant CD4+CD154+ T cells are associated with severe COVID-19, particularly in T cells producing abundant proinflammatory cytokines, including interferon-γ (IFNγ), tumor necrosis factor, interleukin-2 (IL-2) and/or interleukin-21 (IL-21) [58,59]. The gene discussed is IL21; the disease is COVID-19.