C4B and Alzheimer disease: In two loci genome-wide associated with AD, structural variants have also been reported to be associated with disease risk: an 8-kb insertion in CR1 that creates the CR1-S isoform harboring an extra set of C3b/C4b binding sites [10], and a 44 base-pair frameshift deletion in ABCA7 that increases AD risk in African Americans and Caribbean Hispanics [11].