Cathepsin D is an acid protease active in intracellular protein breakdown. Cathepsin D is involved in the pathogenesis of several diseases such as breast cancer and possibly Alzheimer disease. Cathepsin D consists of a light chain and a heavy chain. Defects in CTSD are the cause of neuronal ceroid lipofuscinosis type 10 (CLN10), also known as neuronal ceroid lipofuscinosis due to cathepsin D deficiency. Neuronal ceroid lipofuscinoses are progressive neurodegenerative, lysosomal storage diseases characterized by intracellular accumulation of autofluorescent liposomal material. This results in seizures, dementia, visual loss, and/or cerebral atrophy.