Table 1.

Function of the SLC6A20, LZTFL1, CCR9, CXCR6, XCR1, FYCO1, and ABO genes.

Gene	Description *
SLC6A20	This gene encodes the protein sodium–amino acid (proline) transporter 1 (SIT1), which interacts with the angiotensin-converting enzyme 2 (ACE2)—the SARS-CoV-2 cell-surface receptor—allowing its heterodimerization [19]. The heterodimerization of the ACE2 protein is necessary for the formation of a quaternary structure that functions as a binding site for the SARS-CoV-2 protein S [27].
LZTFL1	The LZTFL1 gene encodes the leucine zipper transcription factor-like 1, and its function is related to tumor-suppressor action and negative regulation of the hedgehog signaling pathways. This gene has high expression in lung tissues [25,28]; it is related to the functioning of the cilia of the pulmonary epithelium and to the signaling of important intracellular pathways, regulating the epithelial–mesenchymal transformation [29].
CCR9	CC chemokines are mainly responsible for the recruitment of lymphocytes. CCR9 is the receptor for the C-C chemokine ligand 25 (CCL25). The CCR9 receptor is mainly found on immature T lymphocytes and the surface of intestinal cells [30]. Animal studies have shown that the CCR9/CCL25 complex participates in the action of T helper 1 (Th1) cells. Another finding indicates that in knockout rats there was a reduction in the mRNA levels of pro-inflammatory cytokines (i.e., IL-6, IL-1β, and TNF-α) [30,31].
CXCR6	CXC chemokines have the highest ability to attract neutrophils and monocytes (30). CXCR6 is the receptor for CXCL16; in cellular studies and animal models, it has been shown to regulate inflammatory activity and influence the levels of INF-γ and TNF-α secreted by CD4+ T cells [32,33].
XCR1	XCR1 encodes the receptor of the XCL-1 ligand. The receptor triggers chemotactic signals in the presence of the ligand [34]. XCR1 is expressed in the lung tissue. Further reports suggest that XCL1 expression in NK cells and CD8+ T cells is constitutively detectable at a steady state, and is elevated during viral infection in mice and humans. The XCL1–XCR1 axis is important for efficient cytotoxic immune response mediated by CD8+ T cells [35].
FYCO1	This gene is responsible for the production of a Rab7 adapter protein, and has the function of assisting in the intracellular transport of autophagic vesicles via transport by microtubules. To carry out the transport, the encoded protein interacts with Rab7 GTPase, phosphatidylinositol-3-phosphate (PI3P), the autophagosome marker LC3, and the kinesin KIF5 [36,37]; it was previously found to be related to inclusion body myositis (IBM) and autosomal recessive congenital cataracts (CATC2) [38,39].
ABO	The ABO gene encodes the enzyme alpha 1-3-galactosyltransferase, which transforms the H antigen expressed on the cell surface of several cell types into A and B antigens. Furthermore, the enzyme converts the H antigen into the von Willebrand factor [40]. Studies indicate that group A confers risk of developing severe forms of infection, while group O confers protection [1]. This effect is related to the expression of anti-A and anti-B antibodies that could neutralize the interaction of the virus protein S with ACE2, blocking its adsorption [41]. Another hypothesis would be its action in the formation of the von Willebrand factor and its relationship with its expression in the pulmonary endothelium, indirectly influencing pro-inflammatory regulation and cell adhesion [42,43].

* The gene functions related to COVID-19 are hypotheses raised by other authors.