Cystatin F (also known as CST7, CMAP or leucocyte cystatin) is an endogenous cysteine protease inhibitor belonging to type II cystatin superfamily. Covalently linked homodimer is formed through C26-C63 disulfide bonds, which is the main form of its secretion. It contains N- glycosylation sites (Asn62 and Asn115), and glycosylation modification determines its intracellular localization-high mannose type guides it to target lysosomes, and complex sugar chain type promotes secretion. The active form is monomer, and the dimer needs to be activated in lysosomal reduction environment, and the 15 amino acids at the N-terminal are removed by cathepsin V to become a completely active form. Mainly expressed in peripheral blood cells (such as cytotoxic lymphocytes, eosinophils, neutrophils, mast cells) and spleen.