GPR15 and chronic obstructive pulmonary disease: Earlier studies have shown site-specific DNA methylation during steroid resistance in COPD, in smokers even after cessation (e.g., Factor II receptor-like 3 (F2RL3) and G-protein-coupled receptor 15 (GPR15)), and in newborns affected by maternal smoking (in utero, or early life exposure) leading to complications later in life [9, 10, 16].