MUC5AC and in situ carcinoma: In a recent review [1], diagnostic criteria of PMCN have been proposed based on well-defined morphologic features including: 1) solitary, well-circumscribed tumors with a cystic gross appearance; 2) no evidence of carcinoma in situ in the bronchial mucosa; 3) mucin comprising greater than 90% of the tumor bulk; 4) neoplastic epithelial cells are mucin-producing and are found lining the fibrous wall or floating within mucin; and 5) no evidence of another primary mucinous malignant neoplasm.