MT1G and chronic obstructive pulmonary disease: Furthermore, marker genes for the MT1G-high non-alveolar macrophage Cluster 3 and the RGS1-high Cluster 6 matched those identified in BAL fluid of healthy controls and patients with COVID-19 [17] (Supplementary Figure S8), while the SERPINB2-high Cluster 13 matched marker genes from BAL cells obtained in healthy and COPD lungs [26] (Supplementary Figure S9).