Despite its initial identification in neurons exposed to the seizure-inducing drug pentylentetrazole (PTZ), the function of the seizure-related gene SEZ-6 remains obscure. Expression analysis indicates specific expression in the adult brain and testis, and the structure of the predicted protein suggests putative roles in cell-cell recognition and signalling. We report here that type I SEZ-6 mRNA is strongly expressed in highly specific regions in the developing forebrain. Specifically, the pattern of SEZ-6 expression is closely tied with the emergence of the neocortical layers and hippocampus, and implies a forebrain-specific role for this gene during development. In the adult hippocampus, SEZ-6 appears to be a CA1-specific regional marker.