Dual mutation of GFAP and vimentin, a fellow intermediate filament protein, causes a small reduction in the extension of cellular processes of reactive astrocytes, whereas normal astrocytes are not affected . These proteins are thought to be effectors mediating hypertrophic remodeling. However, we found that the morphological expansion of astrocytes was independent of GFAP expression during injury-induced reactive responses. Injection of AAV-TRE-YFP into injured Mlc1-tTA cortices caused reactive astrocytes to be labeled with YFP fluorescence (Figure 2f). Specificity of YFP-labeled astrocytes and reactive astrocytes was further verified by co-labeling with antibodies against glutamine synthetase (GS) and nestin, respectively (Supplementary Figure S4a and b). Two and a half days after injury, YFP+ cells were hypertrophic and positive for Acsbg1 (Figure 2f). GFAP did not appear at this stage in every reactive astrocyte, whereas all YFP-labeled astrocytes in the injured cortices exhibited expanded sizes (Figure 2g). Especially, many hypertrophic astrocytes expressed little GFAP (Figure 2g, white arrowheads). Thus, morphological remodeling was likely a reactive response that preceded the onset of GFAP protein expression in astrocytes.