Various plasma phosphorylated tau species have been shown to be associated with amyloid-β (Aβ) PET and Tau PET in Alzheimer’s disease (AD), but whether APOE ε4 affects the interaction between glial fibrillary acidic protein (GFAP) and phosphorylated tau (pTau), and whether a three-way interaction exists among APOE ε4, GFAP, and pTau that influences AD progression remain unclear. The gene discussed is GFAP; the disease is early-onset autosomal dominant Alzheimer disease.