GRN and frontotemporal dementia: Heterozygous GRN mutations cause the deficiency in progranulin (PGRN), a lysosomal and secreted protein, and contribute to the development of FTD,337 while the homozygous GRN mutation leads to neuronal ceroid lipofuscinosis.338 PGRN is expressed in microglia and neurons and shows higher expression once microglia respond to the changes in their CNS environment.