MMP13 and cancer: Among the 28 differentially expressed genes in the bronchus, 3 were up-regulated in patients with cancer: MMP13, a metalloproteinase known to increase lung cancer invasion and metastasis [39]; EDA2R, a member of the tumour necrosis factor (TNF) receptor superfamily, members of which modulate immune response in the tumour microenvironment [40]; and CTSL, a lysosomal cysteine protease involved in epithelial-mesenchymal transition [41].