Acid protease active in intracellular protein breakdown. Plays a role in APP processing following cleavage and activation by ADAM30 which leads to APP degradation (PubMed:27333034). Involved in the pathogenesis of several diseases such as breast cancer and possibly Alzheimer disease. Specificity similar to, but narrower than, that of pepsin A. Does not cleave the 4-Gln-|-His-5 bond in B chain of insulin. Consists of a light chain and a heavy chain (PubMed:8393577, PubMed:1426530). Interacts with ADAM30; this leads to activation of CTSD (PubMed:27333034). Interacts with GRN; stabilizes CTSD; increases its proteolytic activity (By similarity). Identified by mass spectrometry in melanosome fractions from stage I to stage IV. In aortic samples, detected as an extracellular protein loosely bound to the matrix (PubMed:20551380). Expressed in the aorta extracellular space (at protein level) (PubMed:20551380). Expressed in liver (at protein level) (PubMed:1426530). N- and O-glycosylated. Undergoes proteolytic cleavage and activation by ADAM30. As well as the major heavy chain which starts at Leu-169, 2 minor forms starting at Gly-170 and Gly-171 have been identified (PubMed:1426530). An additional form starting at Ala-168 has also been identified (PubMed:27333034). The Val-58 allele is significantly overrepresented in demented patients (11.8%) compared with non-demented controls (4.9%). Carriers of the Val-58 allele have a 3.1-fold increased risk for developing AD than non-carriers. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase A1 family. Neural Ceroid Lipofuscinoses mutation db