APOE and early-onset autosomal dominant Alzheimer disease: The apolipoprotein E4 (ApoE4 [OMIM 107741]) allele is the major genetic risk factor for late-onset Alzheimer disease (AD).1 However, not all ApoE4 carriers develop AD, even among those older than 90 years.2 It is likely that a complex interaction of genetic vulnerabilities with environmental risk factors lead to AD and identifying such factors could be beneficial for the prevention of AD.