Dementia is a syndrome and not a single disease. Approximately 0.5% of those with Alzheimer's disease have an autosomal dominant inherited early onset Alzheimer's disease, caused by mutations in the APP, PSEN1 or PSEN2 gene. A large population-based twin study of late onset Alzheimer's disease supports complex inheritance. The APOE epsilon4 allele is a major risk factor for late onset Alzheimer's disease, whereas the epsilon2 allele has a protective effect. Two large size genome-wide association studies from two Internationals study groups recently identified the genes CLU, PICALM and CRI to be important for late onset Alzheimer's disease. Stroke is like dementia a syndrome and not a single disease. CADASIL is the most common autosomal dominant inherited cause of stroke and vascular dementia. CADASIL is caused by mutations in the NOTCH3 gene, which encodes a single-pass transmembrane receptor. Stroke can cause dementia, as it is the stroke itself rather than the underlying vascular risk factors that cause the dementia.