Among these disorders, a monogenic sub-group has been identified. In Alzheimer's disease, it is associated with the APP, PSEN1 and PSEN2 genes, which account only for 75% of the familial forms with early onset. In frontotemporal dementias, the tau gene mutations account only for 10% of the cases with an autosomal dominant inheritance. The identification of familial forms with a genetic inquiry in the relatives is essential for a greater knowledge of the molecular bases of forms not caused by the known genes, using linkage approaches and candidate gene analysis. The familial forms are also useful for identifying the modifier genes.