To confirm that the detected GFAP+ cells were indeed astrocytes in the cerebral cortex, we costained the cortical slices with antibodies to GFAP and Acsbg1. Acsbg1 immunoreactivity was morphologically reminiscent of the staining observed in the cortical astrocytes of control mice and colocalized well with increased GFAP in Mlc1-ErbB2V664E cortices (Figure 4c). Moreover, Acsbg1+GFAP+ astrocytes possessed enlarged cell bodies and broadened cellular processes, indicating a hypertrophy (Figure 4c). Remarkably, the increased number of GFAP+ astrocytes was due to both GFAP expression in normally GFAP-negative astrocytes and active astrocyte proliferation, as indicated by about half (60.95±4.00% in the cortex, 53.16±4.96% in the corpus callosum and 59.38±12.7% in the midbrain) of GFAP+ cells being immunopositive for Ki67 (Figure 4d).