The identification of mutations in the gene encoding the microtubule associated protein tau in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) demonstrated that tau dysfunction can lead to neurodegeneration. At least 11 missense mutations and 1 deletion mutation (delta K280) have been identified in exons 9-13 that encode the microtubule binding domains of tau. In addition, five mutations have been found close to the 5' splice site of exon 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 highly penetrant mutations that are found in large families with FTDP-17, a common extended haplotype in the tau gene also appears to be a risk factor in the development of the apparently sporadic tauopathy, progressive supranuclear palsy (PSP). The mechanism by which this common variability in the tau gene influences the development of PSP is unclear; however, it further suggests a central role for tau in the pathogenesis of several neurodegenerative conditions including Alzheimer's disease (AD).