While homozygous loss-of-function GRN mutations are very rare and result in a specific subtype of neuronal ceroid lipofuscinosis (CLN11), heterozygous mutations in this gene are more common and, in the case of the Portuguese population, constitute the second leading cause of genetic FTLD, a rapidly progressing type of dementia. This evidence concerns the gene GRN and neuronal ceroid lipofuscinosis.