"To define is to limit": perspectives on asthma-COPD overlap syndrome and personalised medicine.

Introduction Asthma and chronic obstructive pulmonary disease (COPD) are both common and usually distinct airway diseases. However, it is now well recognised that many patients, particularly those aged over 55 years, have features of both asthma and COPD [1–5]. This may occur in up to 20% of patients [3]. In recent years there has been intense interest in the coexistence of asthma and COPD, as evidenced by the proliferation of reviews and editorial space dedicated to the topic [1, 3, 6–9]. It has even been given a new name: asthma–COPD overlap syndrome, or ACOS.