ABCG2 and chronic obstructive pulmonary disease: The in situ human gene expression data demonstrate that ABC transporters are i) variably expressed in epithelial cells from different airway generations (top three expression levels - ABCA5, ABCA13, and ABCC5, ii) regulated by cigarette smoke exposure (ABCA13, ABCB6, ABCC1, and ABCC3), and iii) differentially expressed in individuals with COPD and asthma (ABCA13, ABCC1, ABCC2, ABCC9).