Clustering of lung macrophage single-cell data revealed distinct subclusters that were specific to patients with lung fibrosis. In particular, genes that were differentially expressed in monocyte-derived alveolar macrophages included CHI3L1, MARCKS2, IL1RN, PLA2G7, MMP9, and SPP1. To confirm these findings, the authors performed in situ RNA hybridization to localize these putative profibrotic macrophage subtypes using SPP1 and CHI3L1 and confirmed alveolar macrophage heterogeneity in patients with pulmonary fibrosis [19]. Here, MMP9 is linked to pulmonary fibrosis.