CTSL and COVID-19: For patients with COVID-19 who were not admitted to ICU, there was a statistically significant relationship between viral defense genes and CTSL expression, the four predictors (MX1, TRIM25 SAMHD1 and IFITM3) accounted for 65.56% of the variance (adjusted R2 = 0.656, p-value = 8.796–11).