Expression of the Emx-1 homeobox gene is largely restricted to the developing and mature cerebral cortex. To study its function, two lines of mice were generated using gene targeting methods that have a deletion that includes the N-terminal coding region of Emx-1. Mice homozygous for the deletion were viable and fertile and exhibited no obvious behavioral defects. However, 100% of homozygous mice lack most or all of their corpus callosum, the principle fiber tract that connects the left and right cerebral hemispheres. Heterozygotes show partial penetrance for the corpus callosum abnormality. The histology and various molecular properties of the cerebral cortex appear normal in the mutant mice.