Compared with the plasma YKL-40 levels in healthy control subjects (7.08 [6.17–7.99] ng/ml), levels were significantly elevated in patients with asthma (10.33 [8.79–12.17] ng/ml, p < 0.0001) and those with features of ACO (11.13 [9.06–14.07] ng/ml, p < 0.0001) and were highest in patients with COPD (15.23 [12.31–20.72) (Fig. 1a). The gene discussed is CHI3L1; the disease is chronic obstructive pulmonary disease.