Cathepsin D is the major aspartic proteinase of the lysosomal compartment where it functions in protein catabolism. It is a member of the pepsin family of proteinases. This enzyme is distinguished from other members of the pepsin family by two features that are characteristic of lysosomal hydrolases. First, mature Cathepsin D is found predominantly in a two-chain form due to a posttranslational cleavage event. Second, it contains phosphorylated, N-linked oligosaccharides that target the enzyme to lysosomes via mannose-6-phosphate receptors. Cathepsin D preferentially attacks peptide bonds flanked by bulky hydrophobic amino acids and its pH optimum is between pH 2.8 and 4.0. Two active site aspartic acid residues are essential for the catalytic activity of aspartic proteinases. Like other aspartic proteinases, Cathepsin D is a bilobed molecule; the two evolutionary related lobes are mostly made up of beta-sheets and flank a deep active site cleft. Each of the two related lobes contributes one active site aspartic acid residue and contains a single carbohydrate group. Cathepsin D is an essential enzyme. Mice deficient for proteinase cathepsin D, generated by gene targeting, develop normally during the first 2 weeks, stop thriving in the third week and die in a state of anorexia in the fourth week. The mice develop atrophy of ileal mucosa followed by other degradation of intestinal organs. In these knockout mice, lysosomal proteolysis was normal. These results suggest that vital functions of cathepsin D are exerted by limited proteolysis of proteins regulating cell growth and/or tissue homeostasis, while its contribution to bulk proteolysis in lysosomes appears to be non-critical. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).