Topographic mapping of axon terminals is a general principle of neural architecture that underlies the interconnections among many neural structures. The Eph family tyrosine kinase receptors and their ligands, the ephrins, have been implicated in the formation of topographic projection maps. We show that multiple Eph receptors and ligands are expressed in the hippocampus and its major subcortical projection target, the lateral septum, and that expression of a truncated Eph receptor in the mouse brain results in a pronounced alteration of the hippocamposeptal topographic map. Our observations provide strong support for a critical role of Eph family guidance factors in regulating ontogeny of hippocampal projections.I nformation encoding as topographic maps, which reflects the spatial organization of sensory surfaces and body effectors, is a fundamental mechanism of nervous system function (1, 2). Such topographic representation is critically dependent on precise ordering of axon projections, which maintains the spatial organization of neurons in synaptic connections with target neurons. It has been suggested that topographic axon connections allow transfer of spatial information in the sensory systems (3). In the eye, retinal axons form an inverted projection map in the tectum, with dorsal axons projecting to the ventral tectum, and ventral axons to the dorsal tectum. Similarly, retinal ganglion neurons in the temporal and nasal sides send axons to the anterior and posterior regions of the tectum, respectively (4). Comparable mechanisms may also operate in the limbic circuits, where learning, memory, and emotional responses are elaborated. For example, the hippocampus sends axons topographically to the lateral septum, with dorsomedial neurons projecting to the dorsomedial area of the target, and ventrolateral neurons projecting to the ventrolateral septal region (5, 6). Similarity in organizational principles between limbic circuits and sensory systems suggests that learning, memory, and emotions may be governed by the same spatial constraints as sensory information processing.Based on analyses of regenerating retinal axons, Roger Sperry (7) proposed that axons and target neurons carry specific cytochemical identification tags, and that each axon makes synaptic connections only with neurons carrying matching affinity tags. These tags may be distributed as gradients in the projecting and target fields to specify topographic maps. Recent expression and functional studies suggest that ephrins and their receptors serve as the postulated chemoaffinity guidance tags. The ephrins are a family of cell surface molecules that are ligands of the Eph family tyrosine kinase receptors (8). The Eph receptors, EphA3 in the chicken and EphA5͞A6 in the mouse retina, are expressed in a nasal (low) to temporal (high) gradient (3, 9, 10). Complementary to the receptor gradient, the ligands ephrin-A2 and -A5 constitute an anterior (low) to posterior (high) gradient in the optic tectum (9, 11-13). Disruption of the ligand gradient by re...