CXCL12 and COVID-19: Moreover, the THSD4 gene has been related to two chemokine genes (CCL18 and CXCL12) and a TNF family receptor that participates in respiratory tract mucosal immunity and influences lung function, which is affected in moderate and severe cases of COVID-19 [33], as we found that it was related to the 6MWT, the % FEV1, and DLCO.