Abstract: Frontotemporal lobar degeneration (FTLD) is one of the most common causes of early-onset dementia in patients under the age of 65 years. It is a clinically, genetically, and neuropathologically heterogeneous group of neurodegenerative syndromes, causing atrophy in the temporal and frontal lobes of the brain. This is accompanied by progressive cognitive dysfunction, behavioral changes, difficulties in understanding or producing speech, and often also neuropsychiatric symptoms (Haapasalo and Remes, 2015). Moreover, the clinical, genetic, and neuropathological features of FTLD may overlap with those of amyotrophic lateral sclerosis (ALS). Motor dysfunction is commonly present in FTLD patients and a portion of ALS patients experience frontal and temporal lobe dysfunction (Smith et al., 2019). In addition to the sporadic forms of FTLD, which are not linked to any known mutations, approximately half of the FTLD cases are caused by different mutations in several genes, including GRN (granulin), MAPT (microtubule-associated protein tau), VCP (valosin-containing protein), TARDBP (TAR DNA-binding protein 43 kDa; TDP-43), FUS (fused in sarcoma) and C9orf72 (chromosome 9 open reading frame 72). Of these, the GGGGCC hexanucleotide repeat expansion in the C9orf72 gene (C9-HRE) is the most common genetic cause of both FTLD and ALS (Haapasalo and Remes, 2015). The expansion length can typically vary from hundreds to thousands of repeats in affected individuals.