GRN and neuronal ceroid lipofuscinosis: Early classification of the NCLs centered on the disease age of onset (infantile, late infantile, juvenile, and adult), but this is now supplanted by genetic classification based on discovery of loss-of-function mutations in genes that cause NCL: PPT1, TPP1, CLN3, DNAJC5, CLN5, CLN6, MFSD8, CLN8, CTSD, GRN, ATP13A2, CTSF, and KCTD7 [56].