In this study of 1182 UK children and young adults (4-22 years), 50% of those on regular salmeterol experienced asthma exacerbations over a 6-month period, and 18% required inhaled salbutamol at least daily for symptom relief. Indeed, the investigators reported a step-wise increase in the risk of asthma attacks related to each copy of the Arg16 allele on the β2 receptor gene (1.7-fold) in asthmatic children and young adults exposed to regular salmeterol in conjunction with inhaled corticosteroids. This led the investigators to hypothesize that, contrary to the observations on the overall population of children and young adults where salmeterol is superior in efficacy to montelukast at step 3, those possessing susceptible Arg16 β2 receptor genotype may experience better asthma control with the addition of montelukast rather than salmeterol as second-line controller medication, in addition to inhaled corticosteroids. As such the investigators elected to identify from the database those children with two copies of the Arg16 polymorphism [i.e. homozygous Arg genotype (∼15% of overall population) who would potentially be at greatest risk]. The mechanism for worse control with regular salmeterol involves a greater susceptibility to agonist-induced down-regulation and uncoupling of airway β2 receptors and associated sub-sensitivity of response in the Arg16 genotype.