XCR1 and COVID-19: Individuals with the blood group A had a higher risk of severe COVID-19, whereas there was a protective effect for those with the blood group O. The gene cluster on chromosome 3 contains six genes (CCR9, CXCR6, FYCO1, LZTFL1, SLC6A20, XCR1) with potential roles in COVID-19 severity, such as those involved in immune response (CCR9 and CXCR6), as well as in amino acid transport interacting with ACE2 (SLC6A20).