MUC5AC and small cell carcinoma: According to the World Health Organization (WHO) classification, microscopically, these tumors show various growth patterns (acinar, papillary, bronchioloalveolar, solid with mucin production or a mixed pattern) and is classified into nine types: bronchial gland carcinoma, squamous cell carcinoma, adenocarcinoma, adenosquamous carcinoma, small-cell carcinoma, large-cell carcinoma, neuroendocrine tumor, pulmonary blastoma and combined carcinoma [37,38,39].