MUC1 and pulmonary alveolar proteinosis: In our PAP cohort the mean serum KL-6 level (mean, 2049 U/mL) was considerably lower than reported by Inoue et al. in Japanese PAP patients (mean, 6067 U/mL) [3], but similar to the value observed in Chinese PAP patients (mean, 3127 U/mL) [11].