Legumain is a cysteine protease with a single polypeptide chain structure, containing an active site and several substrate binding sites. It is mainly located in lysosomes and endosomes, participating in protein degradation and antigen processing. By specifically cleaving substrate proteins, Legumain is involved in various biological processes such as cell apoptosis, immune response, and modulation of the tumor microenvironment. The expression of Legumain is tissue-specific, and changes or mutations in its gene expression are associated with various diseases, including tumors, autoimmune diseases, and neurodegenerative diseases.