In autosomal dominantly inherited AD, processing of APP creates a high ratio of Aβ42 (‘long’ Aβ of 42 amino acids) to Aβ40 (‘short’ Aβ of 40 amino acids). ese processes are discussed later in this chapter. e more insoluble long Aβ is the primary constituent of the amyloid plaque. However, immunocytochemical techniques have demonstrated that diuse plaques contain not only Aβ42 but also Aβ40 and, in a small proportion of plaques (most of which have a dense amyloid core), Aβ(16-40/42) (the p3 fragment) (Dickson, 1997). Perivascular amyloid identied in the congophilic angiopathies predominantly consists of Aβ40 (Suzuki et al., 1994; Barelli et al., 1997).