APOE and Alzheimer disease: APOE is important for clearance of amyloid-beta from the brain (Huang and Mucke, 2012), while the AD risk-associated ε4 allele has been shown to impair the clearance of Aβ (Deane et al., 2008) and, more recently, to affect the integrity of the blood–brain barrier (Bell et al., 2012).