Defects in C9orf72 are the cause of frontotemporal dementia and/or amyotrophic lateral sclerosis (FTDALS). An autosomal dominant neurodegenerative disorder characterized by adult onset of frontotemporal dementia and/or amyotrophic lateral sclerosis in an affected individual. There is high intrafamilial variation. Frontotemporal dementia is characterized by frontal and temporal lobe atrophy associated with neuronal loss, gliosis, and dementia. Patients exhibit progressive changes in social, behavioral, and/or language function. Amyotrophic lateral sclerosis is characterized by the death of motor neurons in the brain, brainstem, and spinal cord, resulting in fatal paralysis. Caused by a large expansion of a GGGGCC hexanucleotide within the first C9orf72 intron located between the first and the second non-coding exons. The expansion leads to the loss of transcription of one of the two transcripts encoding isoform 1 and to the formation of nuclear RNA foci. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.