MUC5AC and chronic obstructive pulmonary disease: Consecutive exposure to 1% O2 for 9 days followed by exposure to 21% O2 for another 9 days significantly decreased FOXJ1 expression in both the differentiated HBE cells derived from healthy donors (i.e., normal HBE or NHBE cells) and the differentiated HBE cells derived from COPD subjects (i.e., diseased HBE or DHBE cells) but significantly increased MUC5AC expression in the differentiated DHBE instead of NHBE cells.