One study demonstrated an association between PET Aβ and CSF Aβ40 levels (as well as CSF Aβ38 and a combination of Aβ40 and Aβ38), although the association was stronger in individuals who were not carriers of the apolipoprotein-E (APOE) ε4 allele (a major genetic risk factor for AD) compared with APOE ε4-positive individuals [243]. This evidence concerns the gene APOE and Alzheimer disease.