Cystatin Cystatin(CST)is a class of proteins that inhibit cysteine proteases and are widely distributed in human body fluid and secretion.The present study shows that the CST superfamily is closely related to the tumor,in which the cysteine protease inhibitor SN is the product expressed by the CST1 gene and is abnormal expression in various tumors.However,its occurrence and development of tumor as well as effects of invasion and metastasis on the specific mechanism is not yet clear.In this paper,we retrospectively analyze the related studies in recent years and review the progress of CST1 gene in tumor.