TWIST1 and chronic obstructive pulmonary disease: The role of this key EMT-related transcriptional factor cluster (β-catenin, Snail1 and Twist) specifically in development and progression of airway remodelling in smokers/COPD is still largely unexplored, but our hypothesis is that this system is intimately involved in EMT activity in the airways and its down-stream pathophysiological consequences, as well as having cross-relationship with the SMAD pathway system.