Testing the capability of this ratio to predict the COVID-19 stage by ROC curves (Figure 2), we found the MMP-9/BDNF could be a suitable marker for differentiating stages I/II (AUC 0.7597), stage I/III (AUC 0.9011), and stage I/IV (AUC 0.7727). The gene discussed is MMP9; the disease is COVID-19.