A widely cited example of the usefulness of the candidate gene approach involves AD, the most common cause of dementia in the elderly. AD typically is a late-onset disorder (i.e., the earliest symptoms occur after age 60) with a complex inheritance pattern. The disease often appears to occur sporadically, even when there is an underlying genetic predisposition. One of the pathologic hallmarks of AD is the presence of microscopic aggregates, or plaques, of a small protein-like molecule called β-amyloid peptide. These β-amyloid plaques also contain several other proteins, including one called apolipoprotein E (ApoE), whose gene (APOE) is located on chromosome 19. ApoE was implicated in the development of AD by findings that it binds tightly to β-amyloid in the fluid surrounding the brain and spinal cord (i.e., the cerebrospinal fluid) (Strittmatter et al. 1993). Furthermore, prior linkage data had indicated that a gene for late-onset AD was located on chromosome 19 (Pericak-Vance et al. 1991), in a region that included the APOE gene. Based on these findings, researchers conducted an association study comparing the frequency of three APOE alleles
