Short for apolipoprotein E, a protein involved in Alzheimer's disease
and lipid metabolism.
Different alleles of the apoe gene on chromosome 19
involve different risks for Alzheimer's disease. The epsilon-3 allele is
the most common and is thus associated with normal risk for Alzheimer's and heart disease.
If you have one or more epsilon-4 alleles, you're at higher risk for
both Alzheimer's and heart disease. If you have one or more epsilon-2 alleles, you're at
lower risk for Alzheimer's disease, but at greater risk for
hyperlipidemia and heart disease. (IIRC, the epsilon-1 allele never
shows up in humans.) In this case, a harmful allele doesn't guarantee
that you'll get Alzheimer's; it's not like Huntington's chorea, in which
the wrong allele is an inescapable death sentence.