To confirm the specificity of Mlc1-tTA, an adeno-associated virus (AAV) harboring a TRE-yellow fluorescent protein (YFP) reporter was stereotaxically injected into the brain of 1-month-old Mlc1-tTA mice (Figure 1a). To avoid confusion with a reactive response, Mlc1-tTA targeting cells in the normal brain were identified by YFP fluorescence 1 day after virus injection. YFP-positive (YFP+) cells in the hippocampal hilus were immunopositive for GFAP (Figure 1a). It is now well known that astrocytes in the brain are heterogeneous. Although GFAP, an intermediate filament protein, is a reliable marker for astrocytes in the white matter, cerebellum and hippocampus of the adult brain, it is normally undetectable in the cerebral cortex [7, 11, 25]. Therefore, to confirm the specificity of YFP+ cells in the cerebral cortex, we immunostained AAV-injected cortical slices with an antibody to Acyl-CoA synthetase bubblegum family member 1 (Acsbg1), a protein marker for a wider spectrum of astrocytes [11, 26]. YFP+ cells in the cortex were positive for Acsbg1 (Figure 1a). In addition to adult astrocytes, Mlc1-tTA also targeted astrocyte-like adult neural stem cells (NSCs), as indicated by the presence of YFP-labeled cells coexpressing the NSC marker, nestin (Supplementary Figure 1a and b). Nevertheless, nestin+ cells in the normal adult brain were strictly localized in NSC niches, such as the subgranular zone, subcorpus callosum zone and subventricular zone.