Genetic predisposition is another major risk factor for AD, and
most cases of AD are polygenic, although no genetic etiologies
can be identified in many AD cases.19 The e4 allele of
apolipoprotein E is associated with an increased risk of late-
onset AD in both familial and sporadic cases; however, carrying
1 or 2 apoe4 alleles does not necessarily translate into the
development of AD. Causative mutations in chromosome 21 (amyloid
precursor protein gene), chromosome 14 (presenilin 1 gene), and
chromosome 1 (presenilin 2 gene) have been identified for early-
onset AD, although the proportion of early-onset AD cases caused
by these mutations is debatable. Many other genes and
chromosomes are under investigation.18,19 AD is a heterogeneous
disorder with many etiologies that involve different
interactions between various genetic and environmental risk
factors.3 Clinical features include an impaired ability to learn
new information or to recall previously learned information,
with aphasia, apraxia, agnosia, or distu