Fließer et al investigated the expression levels of ACE2 and other cofactors on both a gene and protein level in lung tissue samples, demonstrating upregulation of ACE2 and TMPRSS2 expression in both bronchial and alveolar epithelial cells, whereas Furin and BSG were downregulated in COPD patients, as compared to controls [36]. Here, BSG is linked to chronic obstructive pulmonary disease.