CTSD and neurodegenerative disease: The CTSD protein levels and activities have indeed been shown to be reduced in PD patients,58, 59 although it has not been unanimously agreed upon yet.39, 58 We speculate that the proper control of expression of lysosomal hydrolases, perhaps through the activation of transcription factor EB, is important for maintenance of proteostasis, and therefore, for the therapy for neurodegenerative diseases.60 Furthermore, genetic and epigenetic changes in genes encoding lysosomal proteins might represent common risks for PD and other neurodegenerative diseases.