The Eph subfamily represents the largest group of receptor protein tyrosine kinases identified to date (1–3). While the biological activities of these receptors have yet to be determined, there is increasing evidence that they are involved in central nervous system function and in development (1–3). The Eph subfamily receptors of human origin (and their murine/avian homologs) include EphA1 (Eph), EphA2 (Eck), EphA3 (Hek4), EphA4 (Hek8), EphA5 (Hek7), EphA6 (Hek12), EphA7 (Hek11/MDK1), EphA8