Stoichiometric transcription factor partnerships control GABAergic neuron fate allocation

Combinatorial transcription factor (TF) codes specify neuronal fates, yet how quantitative differences in interacting TFs shape these decisions remains unresolved. We address this in the developing basal ganglia, where a pool of undifferentiated progenitors gives rise to both D1 and D2 medium spiny neurons (MSNs). Combining sparse in vivo CRISPR perturbation, lineage barcoding, and single-cell transcriptomics in mice, we find that loss of Sp9 shifts clonal fate bias from D2 toward D1 MSNs and intercalated cells. Mechanistically, chromatin profiling and biochemical assays show that at GC-rich promoters, SP9 binds DNA directly and activates transcription. At distal enhancers, SP9 binds indirectly, tethered by DLX factors, represses activity, and associates with the NuRD corepressor complex. The relative abundance of SP9 and DLX selects between these modes. These findings extend the combinatorial TF code beyond factor identity to relative proportions, with relevance to neurodevelopmental disorders.