However, TGF-β is well known to promote airway smooth muscle differentiation and accumulation109–112, and erythropoietin-induced TGF-β secretion could conceivably represent the key pathogenetic contributor that promotes the emergence of asthma in patients with the comorbidity background of subgroup 6, in which COPD is the most frequent comorbid disease. This evidence concerns the gene TGFB1 and asthma.