Mutations in GUSB, CLN6, and PPT1 cause Sly disease, neuronal ceroid lipofuscinosis (CLN6), and neuronal ceroid lipofuscinosis (CLN1), respectively, whereas mutations in GALNS and NAGA cause Morquio A disease and Schindler disease/Kanzaki disease, respectively. The gene discussed is PPT1; the disease is mucopolysaccharidosis type 4.