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als are unaffected. Genome-wide mapping provided suggestive linkage at two loci on chromosomal arms 3p and 15q. Alleles at either locus alone were not sufficient to explain FCD; however, considered together, both loci could explain the disorder in this pedigree. Subsequent next-generation sequencing identified a nonsense mutation in AGBL1 in the 15q locus; this mutation would result in a premature termination of AGBL1. Consistent with a causal role for this transcript, further sequencing of our cohort of late-onset-FCD-affected individuals identified two cases harboring the same nonsense mutation and a further three unrelated individuals bearing a second missense allele. AGBL1 encodes a glutamate decarboxylase previously identified in serial analysis of gene expression of corneal endothelium, a finding confirmed by immunohistochemical staining. Wild-type AGBL1 localizes predominantly to the cytoplasm; in sharp contrast, the truncated protein showed distinct nuclear localization. Finally, we show that AGBL1 interacts biochemically with the FCD-associated protein TCF4 and that the mutations found in our cohort of FCD individuals diminish this interaction. Taken together, our data identify a locus for FCD, extend the complex genetic architecture of the disorder, provide direct evidence for the involvement of TCF4 in FCD pathogenesis, and begin to explain how causal FCD mutations affect discrete biochemical complexes.