GRM1 and Huntington disease: The Synapse Assembly cluster (Figure 2B) was significantly enriched in genes with reduced expression in HD OPCs (Fisher’s exact test, p<0.001), including SYT13, LRRTM1, GRM1, EPB41L2, DLGAP3, and AGAP2; the only gene of this cluster that was upregulated in HD OPCs was NRXN3 (Figure 3B, C).