In the last 10 years significant progress has been made to describe and identify the underlying biological mechanisms that cause the different manifestation of Alzheimer's disease. Since the first report of a possible locus on chromosome 21 in a small group of families with early onset familial Alzheimer's disease (FAD), considerable progress has been made. Results from linkage analysis and gene sequencing has provided evidence that a minority of early onset FAD families develops the disease as a result of mutations in the gene coding for the Abeta-amyloid precursor protein, and that mutations in presenilin 1 and 2 genes account for a larger subgroup of early onset families. Several other early onset FAD families are clearly not linked to any of these loci, suggesting that other genetic risk factors may exist. Recent genome-wide scanning studies have revealed the existence of a new locus on chromosome 12, which, together with inheritance of the epsilon4 allele of apolipoprotein E gene, on chromosome 19, represent the most important genetic factors associated with an increased risk of developing the disease in late onset FAD families.