Search terms included "mucus plugs," "mucus plugging," "biologics," "dupilumab," "tezepelumab," "mepolizumab," "benralizumab," "IL-4," "IL-13," "MUC5AC," "quantitative CT," "functional respiratory imaging," "asthma," and "COPD." Studies were included if they reported original data or systematic evidence on mucus plug quantification, biologic-mediated changes in mucus plug scores, or imaging modalities for mucus assessment in asthma or COPD. The gene discussed is IL4; the disease is chronic obstructive pulmonary disease.