Now is a great time to think about the genetics of AD. Since the demonstration of the gene for the AD amloid precursor protein on chromosome 21 ( gene dosage appears to be the explanation for the heightened risk of AD in trisomy 21 individuals who live into early adulthood), the list has expanded to include loci on chromosomes 1 and 14 in early onset families. The apolipoprotein E (APOE)-4 allele has been identified as a risk factor in both late onset familial and sporadic AD. I expect the list will continue to grow and that we will see combinations of genetic risk factors. Is this one disease?