GRN and neuronal ceroid lipofuscinosis: While inheritance of a single GRN mutation causes FTD, patients with homozygous loss‐of‐function mutations present much earlier in life with neuronal ceroid lipofuscinosis (NCL, Batten disease), which is characterized by the accumulation of auto‐fluorescent material (lipofuscin) in the lysosomes of neurons, rapid cognitive decline, and retinal degeneration.3