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The clinical syndrome of frontotemporal dementia is associated with several neurodegenerative disorders: Pick's disease, corticobasal degeneration, motor neuron disease-associated dementia (MND-dementia), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), and frontal lobe degeneration (FLD). These disorders, although they do not match the frequency of Alzheimer's disease, are far from uncommon, and present clinicians and neuropathologists with formidable, if not insurmountable diagnostic difficulties. However, recent advances in cellular and molecular pathology, biochemistry, and molecular genetics have been instrumental in their nosological definition. The discovery of a mutation in the tau protein gene on chromosome 17 in 1998 has established that several phenotypically heterogeneous familial dementias with a confusing variety of names all belong to FTDP-17. 1 2 Most but not all frontotemporal dementias are characterised by intracellular inclusions formed by abnormal cytoskeletal components, both in neurons and in glial cells. Pick's disease, corticobasal degeneration, and FTDP-17 belong to the larger group of tauopathies, as their hallmark lesions contain tau protein, distinguishing them from MND-dementia and FLD, two disorders without tau...