Neuronal ceroid lipofuscinosis type 2 (CLN2) disease, also known as late infantile neuronal ceroid lipofuscinosis, is one of the most common NCLs and is caused by pathogenic variants in the TPP1 gene encoding the lysosomal enzyme tripeptidyl peptidase 1 (TPP1). The gene discussed is TPP1; the disease is neuronal ceroid lipofuscinosis 2.