MUC1 and pulmonary alveolar proteinosis: Serum KL-6 levels were significantly higher in PAP patients than in healthy controls (mean ± SEM: 5004 ± 983 vs. 283 ± 19 U/mL, p < 0.0001), and serum LDH levels were also higher in PAP patients than in healthy controls (312 ± 24 vs. 193 ± 6 IU/L, p < 0.0001) (Fig. 1).