The cathepsin family of proteolytic enzymes include several diverse classes of proteases. Cathepsins B, L, H, K, S and O comprise the cysteine protease class. Cathepsins D and E comprise the aspartyle protease class. The serine protease class includes cathepsin G. Cathepsins function in cellular metabolism and participate in peptide biosynthesis and protein degradation. Cathepsin A, a serine carboxypeptidase, exists in a high molecular weight lysosomal complex with b-galactosidase (b-gal) and a-neuraminidase (Neu1). Cathepsin A functions to protect b-gal and Neu1 from intralysosomal proteolysis. Deficiencies in cathepsin A lead to deficiencies in b-gal and Neu1 . The gene encoding human cathepsin A maps to chromosome 20q13.12. Mutations in this gene cause glactosialidosis, a lysosomal storage disorder resulting from the b-gal and Neu1 deficiencies.