Effective medicines are available, but a child's response to these medicines is currently unpredictable. This project focuses on an asthma controller medicine called salmeterol. According to reports, tens of thousands of children may be taking this medicine in the UK, but evidence suggests it might not work for around one in seven of them. The study team are investigating whether a new approach to treatment, where prescribing is personalised according to a child's genetic make-up, improves the child's quality of life and provides better control of their asthma. Treatment that is tailored in this way to a person's genetic features is often called 'personalised medicine'.