In humans, homozygous mutations of CTSD causes neuronal ceroid-lipofuscinosis, an early onset lysosomal storage disease, while heterozygous missense mutations results in early-onset motor and visual disturbances, brain atrophy, and progressive psychomotor problems.44 However, the effects of partial decline of CTSD activity on development and progression of late-onset neurodegenerative disease, such as PD, has not been addressed. Here, CTSD is linked to Parkinson disease.