TPP1 and Lysosomal disease: In the cln2 lysosomal disease model, mutants with knockdown and knockout of the tpp1/cln2 gene, which encodes the lysosomal protein tripeptidyl peptidase I (TPP-I), a soluble lysosomal aminopeptidase, accumulate cellular autofluorescent material (Phillips and Gomer, 2015; Smith et al., 2019).