One of 2 snps which is necessary to identify ApoE4.
Inheritance of the rs1799724(T) allele (a variant in another gene, TNF) appears to synergistically increase the risk of Alzheimer's in APOE4 carriers and is associated with altered CSF Abeta42 levels [PMID 15895461]
[PMID 17192785] The researchers found that on testing DNA samples from 1,086 well-characterized Alzheimer's disease cases, a single SNP (rs4420638) lying 14 kb distal to the ApoE locus has a powerful association with late-onset AD (corrected p value was 5.3 x 10 e-34). No other SNP showed as robust an association. The authors estimated that people with two ApoE4 copies, such as a person with the rs429358(C;C) genotype, have a 25-fold increased risk for developing the disease compared to ApoE3/ApoE3 carriers.