Legumain is a lysosomal cysteine protease which is a member of the peptidase C13 family. Though it is found in many tissues, it is highly expressed in the kidney, heart, and placenta. Legumain has a strict specificity for hydrolysis of asparaginyl bonds and can also cleave aspartyl bonds slowly, especially under acidic conditions. Over-expression Legumain in tumors is significant for invasion and metastasis. In addition, Legumain may be involved in the processing of proteins for MHC class II antigen presentation in the lysosomal/endosomal system and negative regulation of neuron apoptosis.