We have isolated an enhancer-trap mouse line (HXZ 3-13) that expresses a zfp40-lacZ fusion gene mainly in the embryonic limb and the adult hippocampal neuron. In this transgenic line the lacZ expression seems to be under the control of an enhancer element located around the transgene. By screening a mouse BAC library we isolated a 140 kb DNA segment that corresponds to a flanking region of the transgene, which may contain the enhancer element that confers in adult the hippocampus-specific expression of lacZ as well as the transcriptional unit that is under the control of this enhancer. We next identified a novel transcriptional unit on the 140 kb segment. In the HXZ 3-13 line the zfp40-lacZ was inserted in this transcriptional unit, suggesting that the HXZ 3-13 has a functional mutation in this transcriptional unit. The expression was observed mainly in the brain, kidney and skeletal muscle.