To ensure that lysosomal dysfunction was solely due to reduced TPP1 activity, similar methods (fluorogenic enzyme activity assays) were used to assess the activity of palmitoyl-protein thioesterase 1 (PPT1), the lysosomal enzyme associated with cINCL, and sphingomyelinase, the lysosomal enzyme associated with Niemann-Pick disease. Here, PPT1 is linked to Niemann-Pick disease.