MMP13 and chronic obstructive pulmonary disease: Since the full gene expression signature is not available, we were only able to compare the top 5 most significant genes for positive sex-by-COPD interaction (SGMS1, LINGO2, SLC25A1, KDSR, SKP1) and the top 5 most significant genes for negative sex-by-COPD interaction (CYorf15A, ZFY, AMPD3, STAP1, MMP13) against our sex-by-smoking interaction results in airway epithelium.