A definitive diagnosis of primary mucinous carcinoma of the thyroid gland is difficult to establish, as some types of papillary carcinomas, follicular carcinomas, medullary carcinomas, and thyroid metastases from cancers of other organs also produce mucin, which is an important feature of primary mucinous carcinomas of the thyroid gland [5–7]. Here, MUC5AC is linked to medullary thyroid gland carcinoma.