Previous studies have shown that low plasma Aβ1–42 concentrations appear in MCI/AD patients carrying the Apolipoprotein E ε4 allele (APOE4) (Kleinschmidt et al., 2016) and in cognitively normal individuals showing abnormal CSF-amyloid status and positive amyloid PET scans (Verberk et al., 2018). The gene discussed is APOE; the disease is Alzheimer disease.