In patients with a AE-COPD history of ≥2 moderate exacerbations or ≥1 severe exacerbation within the last year compared to patients without AE-COPD and non-obstructive controls irrespective of their smoking history, PARP-1 expression was increased in GOLD grades 1–3 of airflow obstruction and GCLM in GOLD grades 1–2 and 4, HDAC2 expression was decreased in GOLD grades 1–3, DDP4 fell in GOLD grade 4, and GCLC fell in GOLD grades 1–2 (Figure 4d, Supplementary Table S1). Here, GCLC is linked to chronic obstructive pulmonary disease.