MUC1 and obstructive sleep apnea syndrome: In a previous study of 11 OSA patients and 10 controls using Krebs von den Lungen 6 (KL-6), a biomarker for the diagnosis and prognosis of ILD, the KL-6 level was significantly higher in the OSA group than the controls (317 U/mL vs. 226 U/mL, p = 0.03).