As the presence of those counter receptors generally marks cytotoxic effector lymphocytes in peripheral blood (Nishimura et al., 2002), we correspondingly found that CD8+ T cells and NK cells were the major cytotoxic populations expressing perforin and granzymes (Figure 4b), especially in COVID-19 patients (Figure 4—figure supplement 1). This evidence concerns the gene PRF1 and COVID-19.