Frontotemporal dementia and/or amyotrophic lateral sclerosis 1 is an autosomal dominant neurodegenerative disorder with adult onset. It is genetically and pathologically heterogeneous, with significant intrafamilial variation. Patients with C9ORF72 repeat expansions tend to have earlier onset, shorter survival, bulbar symptoms, and increased incidence of neurodegenerative disease in relatives. Psychiatric disturbances may precede dementia onset. The disease involves disturbances in RNA processing, autophagy, and other cellular processes. Frontotemporal dementia with motor neuron disease is characterized by dementia-associated psychiatric symptoms, memory difficulties, and motor neuron manifestations. Frontotemporal dementia is marked by frontal and temporal lobe atrophy, while amyotrophic lateral sclerosis involves motor neuron death leading to paralysis. FTDALS1 is rare and caused by C9ORF72 gene changes, with symptoms including personality changes, cognitive impairments, and motor neuron-related issues. Hexanucleotide repeat expansions in the C9ORF72 gene are associated with FTDALS1, inherited in an autosomal dominant pattern. Symptoms vary widely between and within families, with earlier and more severe presentations in ALS cases.