Membrane-associated guanylate kinases (MAGUKs) contain multiple protein-binding domains that allow them to assemble specific multiprotein complexes in particular regions of the cell(1,2). CASK/LIN-2, a MAGUK required for EGF receptor localization and signalling in Caenorhabditis elegans, contains a calmodulin-dependent protein kinase-like domain followed by PDZ, SH3 and guanylate kinase-like domains(3-5). In adult rat brain, CASK is concentrated at neuronal synapses and binds to the cell-surface proteins neurexin and syndecan(6-8) and the cytoplasmic proteins Mint/LIN-10 and Veli/LIN-7 (refs 4, 9, 10). Here we report that, through its guanylate kinase domain, CASK interacts with Tbr-1, a T-box transcription factor that is involved in forebrain development(11,12). CASK enters the nucleus and binds to a specific DNA sequence (the T-element) in a complex with Tbr-1. CASK acts as a coactivator of Tbr-1 to induce transcription of T-element containing genes, including reelin, a gene that is essential for cerebrocortical development. Our findings show that a MAGUK which is usually associated with cell junctions has a transcription regulation function.