Since the density of the β2-adrenoceptors and the muscarinic M3-receptor increases and decreases, respectively, as the airway size decreases [13,14], the bronchoprotection offered by β2-agonists and anticholinergics could potentially differ for large compared to small intrapulmonary airways which could impact the pharmacological treatment of respiratory diseases. Here, CHRM3 is linked to respiratory system disorder.