Light micrograph section through human tissue showing mucinous gastric carcinoma. Mucinous carcinoma is an invasive type of cancer that begins in an internal organ that produces mucin, the primary ingredient of mucus. The abnormal cells inside this type of tumour are floating in the mucin where the mucin becomes part of the tumour. Mucinous gastric carcinoma (MGC) is a rare histological sub-type of gastric cancer.