Neuronal ceroid lipofuscinosis type 2 (CLN2 disease; OMIM#204500) is an autosomal recessive disease, caused by deficiency of the lysosomal enzyme tripeptidyl peptidase (TPP1; EC 3.4.14.9) [1, 2]. The gene discussed is TPP1; the disease is neuronal ceroid lipofuscinosis 2.