APOE and early-onset autosomal dominant Alzheimer disease: Genome-wide association studies and genome sequencing have identified more than 25 Alzheimer's disease (AD) risk loci, including common, low-risk variants and rare moderate risk variants, in addition to the classical risk variants in the apolipoprotein E (APOE) gene (Guerreiro et al., 2013b, Hollingworth et al., 2011, Jonsson et al., 2013, Lambert et al., 2013, Naj et al., 2011, Pimenova et al., 2018, Steinberg et al., 2015).