Serum KL-6 levels are generally higher in PAP cases than in other IP cases. It has been reported that the cut-off value for serum KL-6 to differentiate APAP from other IPs is 2,050 U/mL [17], though low KL-6 levels do not necessarily exclude the diagnosis of PAP. This evidence concerns the gene MUC1 and incontinentia pigmenti.