The invention discloses a gene MUC21 and application of an expression product of the gene MUC21 in hypophysoma diagnosis and treatment.The gene MUC21 related to generation and development of hypophysoma is screened through a bioinformatics method, function verification is conducted through a fluorescent quantitative method and the like, and down-regulated expression of the gene MUC21 is conducted in hypophysoma patient tissue; by increasing the expression of the gene MUC21, the excessive proliferation of hypophysoma cells can be restrained.The invention further discloses a product for diagnosing hypophysoma and a medicine composition for treating hypophysoma.