In agreement with the previous studies [48, 49], CSM exposure induced MUC5AC overexpression with further upregulation of MUC5AC expression after SARS-CoV-2 infection, suggesting that SARS-CoV-2 might aggravate mucus hypersecretion to cause further airway obstruction in smokers and COPD patients. The gene discussed is MUC5AC; the disease is Airway obstruction.