CysC was the first cystatin found co-localized with Aβ in amyloid-laden vascular walls, and in senile plaque cores of amyloid in brains of patients with AD, Down's syndrome, HCHWA-D, intracranial hemorrhage, cerebral infarction, and of elderly subjects without any neurological disorder (Maruyama et al., 1990; Vinters et al., 1990; Itoh et al., 1993; Haan et al., 1994; Levy et al., 2001). The gene discussed is CST4; the disease is Alzheimer disease.