SUMMARY: Inhibitory interneurons comprise a diverse class of neurons of indisputably high biomedical importance. Interneuron dysfunctions are linked to neuropsychiatric disorders such as autism and schizophrenia and are also implicated in epilepsy and neurodegeneration. Cortical interneurons are born in the ganglionic eminences. As they develop, they migrate to intercalate into diverse cortical circuits throughout the brain, where they become increasingly specialized and modulate many aspects of neural circuit function. Recent transcriptomic and lineage tracing studies have begun to identify new levels of transcriptional diversity among interneuron subtypes and between species. Similarly, genomic studies are increasingly identifying genetic risks for disease that are predicted to impact interneuron function. What is needed to integrate these findings are 1) means to define the genetic programs that endow interneurons with their successive levels of functional specialization and 2) to be able to produce these features in vitro for use in disease modeling and genetic loss and gain-of-function studies. The overall goal of this study is to leverage a new combinatorial transcription factor (TF) screening method for direct neuronal reprogramming to define transcriptional programs that produce both generic and more specialized interneuron subtype identities. We will then test the resulting induced interneurons using deep quantitative methods for cell type phenotyping at the transcriptional, epigenetic, and functional level. We will apply new AI-enabled in vitro methods to assess cell morphology, motility, and synaptic specificity in vitro, paired with in vivo assays for migration and specific synapse formation. These collaborative interdisciplinary studies will accomplish three goals relevant to brain development and repair. First, they will enable the production of defined interneuron subpopulations in vitro, which will accelerate genetic studies using loss and gain-of-fu