Additionally, we found 44, 42, and 53 up-DEGs that were notably associated with mild, moderate, and severe COVID-19, and there were six significant common genes across three phases of COVID-19, including TCF7, GZMH, RAB5IF, CCND2, BIRC6, and NDUFAF3 (Fig. 5J–K and Additional file 3: Fig. S27G-N). The gene discussed is CCND2; the disease is COVID-19.