GRN and hyperinsulinemic hypoglycemia, familial, 4: Progranulin is trafficked to lysosomes9,10 and is necessary for maintaining lysosomal function, as people with loss-of-function mutations on both GRN alleles, resulting in nearly complete progranulin deficiency, develop the lysosomal storage disorder Neuronal Ceroid Lipofuscinosis (NCL)11–14.