For instance, hypomethylation in SERPINA1 (Serpin family A member 1) gene at two CpG sites (cg02181506 and cg24621042 on chromosome 14) was linked to the COPD and smoker group, as well as AHRR (Aryl-hydrocarbon receptor repressor) gene hypomethylation in intron 3 (cg21161138) [43,46]. Here, AHRR is linked to chronic obstructive pulmonary disease.