MUC1 and chronic obstructive pulmonary disease: In the middle aged/elderly subjects, the mean plasma level of KL-6 was higher in both smokers and subjects with COPD when compared to non-smokers (non-smoker 309 ± 35 U/ml, smoker 478 ± 31 U/ml, COPD 589 ± 61 U/ml; p = 0.003 and p < 0.001, respectively).