MUC5AC and lung adenocarcinoma: Pure mucinous adenocarcinoma is defined by the World Health Organization as lung tumor cells comprising goblet and/or columnar cells secreting abundant extracellular mucin and making up more than 50% of the tumor volume; it accounts for only 2%‐5% of all lung adenocarcinomas.3, 4 Several previous reports have stated that the clinicopathological features and prognosis of PMA are unique from those of other histopathological types of lung adenocarcinoma.4 However, most of the previous studies of PMA are limited.