P2RX4 and chronic obstructive pulmonary disease: In case of stratification by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) classification into “GOLD III and IV” and “GOLD I and II” COPD patients, P2RX4 expression was increased in BALF cells and blood MNCs of GOLD III and IV COPD patients while COPD patients classified as GOLD I and II showed no significant difference compared to never-smokers (Additional file 1: Fig. S2).