Modified sindbis virus for MAPseq was obtained from the MAPseq Core Facility at Cold Spring Harbor Laboratory7 (link). The viral library used in this study had a diversity of 20,000,000 unique barcodes. Retro-AAV2 coding for mCherry (pAAV2-hSyn-mCherry, Addgene #114472, 2 × 1013 GC/ml) and green fluorescent protein (pAAV2-hSyn-eGFP, Addgene #50465, 2.2 × 1013 GC/ml) under the human synapsin promoter were obtained from Addgene. All viruses were stored at −80°C and aliquots were thawed over wet ice immediately prior to injection.