TPP1 and neuronal ceroid lipofuscinosis: Mutations in seven known genes (CLN2/TPP1, CLN5, CLN6, CLN7/MFSD8, CLN8, and CLN1/PPT1 or CLN10/CTSD) cause NCL disease with onset in late infancy.2, 3 CLN1, CLN2 and CLN10 diseases can be diagnosed by enzyme assay, the remainder can be distinguished only by sequencing to identify or exclude a disease-causing variation.