GFAP and Alzheimer disease: Therefore, in this study, we utilized two independent cohorts and applied a comprehensive approach combining cross-sectional and longitudinal analyses with both plasma biomarkers and PET imaging to specifically address the following key questions: (1) whether APOE ε4 genotype and plasma GFAP modulate the relationship between plasma pTau and AD pathologies, (2) whether the interaction among APOE ε4, GFAP, and pTau differentially influences AD pathologies.