MUC5AC and squamous cell carcinoma: Prior to the 2015 WHO classification update, adenocarcinoma of the lung was diagnosed based on acinar or tubular patterns, mucin production, and positivity for TTF-1 and Napsin A. p40, the best marker for squamous cell carcinoma (SqCC) in terms of specificity, was only observed in about 3 % of adenocarcinoma cases [9].