TPP1 and neuronal ceroid lipofuscinosis 2: Neuronal ceroid lipofuscinosis type 2 (CLN2) disease is a very rare, severe, neurodegenerative lysosomal storage disorder caused by autosomal recessive inheritance of genetic variants in the TPP1 gene, which encodes for the lysosomal serine protease tripeptidyl peptidase 1 (1–4).