library_construction_protocol: half brains were immediately dissected, the cerebellum removed, and the tissue stored brieﬂy in RPMI. Tissue dissociation was then performed using a Dounce homogenizer. Dissociated cell pellets were treated with Mouse cell removal beads (Miltenyi) to remove all mouse cells. Samples were then separated using LS columns and the Midi MACs separator (Miltenyi). Myelin and debris were removed by resuspending the pellet in 8mL 30% Percoll, overlaid with 2mL of 1X DPBS, Purified human cells were then pelleted via centrifugation (10 min, 400xg, 4°C) and dead/apoptotic cells were removed using the EasySep Dead Cell Removal (Annexin V) kit (Miltenyi; Cat. # 17899). Cell pellets were resuspended in 70uL of Dead Cell Removal (DCR) buffer (2% BSA and 1mM CaCl2 in PBS) followed by the addition of 5uL of the DCR Cocktail and 5uL of the Biotin Selection Cocktail. Samples were then incubated at room temperature for 3 min. After incubation, 10mL of DCR beads were added immediately followed by 2.4mL of DCR buffer and incubated at RT for 10 minutes on the EasyEights EasySep magnet (StemCell Technologies; Cat. #18103). Keeping the sample on the magnet, the supernatant was collected and pelleted by centrifugation (10 min, 400xg, 4°C). Cells were then counted on a hemocytometer and resuspended at ~1,000 total cells per microliter in FACS buffer. High quality and low-debris samples were then pooled to result in 1-2 scRNA-seq samples per genotype. scRNA-seq library preparation was performed according to the 10X Genomics Chromium Single Cell 3' Reagents kit v3 user guide except that sample volumes containing 25,000 cells are loaded onto the 10X Genomics flow cell to capture ~10,000 total cells. Raw bulk RNA-seq data was initially assessed for quality using FastQC 0.11.9, and results summarized with MultiQC 1.10.1. Data then underwent quality trimming and adapter removal using Trimmomatic 0.39.