Detailed Brain Dissection and Histological Analysis

Mice were sacrificed by carbon dioxide asphyxiation or deep anesthesia with isoflurane followed by cervical dislocation. Mouse brains were manually dissected under a fluorescent stereoscope (Zeiss, Olympus or Leica). Brightfield and/or GFP fluorescent images were taken for the dissected brain, and overlaid using ImageJ^{58 (link)}. Brains were then fixed in 4% formaldehyde or 10% formalin for 48 to 96 hours, embedded in paraffin, sectioned at 6 μm and stained with hematoxylin and eosin (H&E) for pathology. For tumor size quantification, H&E slides were scanned using an Aperio digital slidescanner (Leica). Tumors were manually outlined as region-of-interest (ROI), and subsequently quantified using ImageScope (Leica). Sections were de-waxed, rehydrated and stained using standard immunohistochemistry (IHC) protocols as previously^{55 (link), 59 (link)}. The following commonly used antibodies were used for IHC: rabbit anti-Ki67 (abcam ab16667, 1:500), rabbit anti-GFP (ThermoFisher Scientific A11122, 1:300) rabbit anti-GFAP (Dako, 1:500), and mouse anti-Cas9 (Diagenode, 1:300).