MUC5AC and interstitial lung disease: The initiating event that leads to the formation of usual interstitial pneumonia (UIP) remains unknown but hyperplastic ATII cells are often seen at areas of active fibrosis in patients with fibrotic ILD [30] and there is some evidence to suggest that microscopic honeycomb cyst formation has a distal airway origin driven by dysregulation of the mucin gene MUC5B [14].