Therefore, in addition to the ORF gene sequence in SARS-CoV2, AhR activation by KYN may be involved in type I IFN suppression in patients with severe COVID-19 (38), suggesting that activation of the IDO1-KYN-AhR pathway promotes mucus secretion in alveolar epithelial cells and induces hypoxia in patients with COVID-19 (39). This evidence concerns the gene IDO1 and COVID-19.