The identification of mutations in the tau gene in frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) demonstrated that there is a direct link between tau dysfunction and neurodegeneration. At least 11 missense mutations and a three base pair deletion (DeltaK280) have been identified in exons 9-13. Additionally, five splice site mutations have been found in intron 10. The different FTDP-17 mutations have multiple effects on the biology and function of tau. These varied pathogenic mechanisms likely explain the wide range of clinical and neuropathological features observed in different families with FTDP-17. In addition to the tau mutations, a common extended haplotype in the tau gene also appears to be a risk factor in the development of the apparently sporadic tauopathies progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). The mechanism by which this common variability in the tau gene influences the development of these neurodegenerative diseases is unclear; however, it further suggests a central role for tau in the pathogenesis of several neurodegenerative conditions including Alzheimer's disease (AD).