Representative analysis of methylation promoter frequency from sputum samples from CTR, COPD and LC subjects. Methylation promoter frequency (%) for p16, CDH1 and MGMT in COPD and LC group, both compared with CTR group. Group differences were assessed using Fisher’s test. ** P < 0.0001 respect to control group (CTR). * P < 0.05 respect to control group (CTR).