Describes a group of rare familial central nervous system disorders characterized by amyloid deposition in the cerebral blood vessels leading to hemorrhagic and non-hemorrhagic strokes, focal neurological deficits, and progressive cognitive decline eventually leading to dementia. Clinical features depend on the disease type. Most forms of HCHWA (Dutch, Arctic, Piedmont, Iowa, Flemish and Italian) are due to a point-mutation in the APP gene on chromosome 21q21.2, which encodes the beta-amyloid precursor protein. This mutation causes increased accumulation of amyloid-beta protein in the walls of cerebral arteries and capillaries. Only one form of HCHWA, Icelandic type, is due to a mutation in the CST3 gene on chromosome 20p11.2, encoding the precursor protein cystatin C.