CCR9 and COVID-19: Among these genes, 2 genes (APOE and SLC6A20) were associated with COVID-19 susceptibility, 6 genes (LZTFL1, CCR9, FYCO1, CXCR6, XCR1, and ABO) were related to COVID-19 severity, 4 genes (ERAP2, BRF2, TMEM181, and ALOXE3) were associated with COVID-19 mortality, and 5 genes (ACE2, ANPEP, DPP4, ENPEP, and TMPRSS2) were detected in SARS-CoV-2 receptors (Supplementary Table S8).