ISG15 and COVID-19: Through GO analysis of the possible signalling pathways regulated by these sites, it was found that HERV-K (HML-2) gag was related to the interferon signalling pathway, and Pearson correlation analysis showed that HERV-K (HML-2) gag was positively correlated with IFNB1, ISG15, and IFIT1 (Supplementary Figure S1, p < 0.05), which perfectly confirmed the hypothesis that HERV-K (HML-2) activated interferon secretion in COVID-19 patients.