Further analysis of the correlation between invasion genes showed that the epithelial entry factor genes (BSG, FURIN, CTSL) were positively correlated with the ratio of early EMT in COVID-19, TMPRSS2 was negatively correlated with the ratio of early EMT in COVID-19, and FURIN was positively correlated with the ratio of early EMT in IPF (Figure 3J). The gene discussed is CTSL; the disease is COVID-19.