Smoking induces epigenetic changes in COPD patients, and genome-wide association studies (GWAS) have provided compelling evidence on the association between polymorphisms in CYP2A6, a nicotine metabolising enzyme, on chromosome 19 and nicotinic acetylcholine receptors (CHRNA3/CHRNA5/CHRNAB4) cluster on chromosome 15 with the number of cigarettes smoked per day both in the general population and COPD patients [36, 37]. Here, CYP2A6 is linked to chronic obstructive pulmonary disease.