CTSB and chronic obstructive pulmonary disease: Additional genes of interest that were substantially upregulated and have direct relevance to COPD and/or cigarette smoke exposure included SERPINA1 (2.17 log2 fold change (log2FC); padj = 5.13E−26), ATP6V0D2 (3.99 log2FC; padj = 3.42E−26), MMP-12 (2.69 log2FC ; padj = 6.20E−12), AHRR (1.74 log2FC; padj = 8.32E−05), and CTSB (1.83 log2FC; padj = 4.64E−18), among others [4, 50].