CYP1B1 and chronic obstructive pulmonary disease: These identified genes with the greatest differences in expression are known to be associated with smoking, including gene families mediating the metabolism of xenobiotic substances [56], such as AHRR, CYP1B1, and CYP4F3. These genes are involved in oxidative stress upregulated by smoking exposure [57] and are associated with lung diseases such as COPD [58].