GFAP and Alzheimer disease: While GFAP, which is used as a broad marker of reactivity, has helped correlate changes in astrocytes with AD pathology in vivo (e.g., Aβ plaques), a disadvantage of using GFAP alone remains, namely that astrocytes maintain heterogeneous levels of GFAP across brain regions, development, and in multiple reactive sub-states [22,126,127].