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The erythropoietin-producing hepatocellular (Eph) family of ligands and receptors has been implicated in the control of axon guidance and the segmental restriction of cells during embryonic development. In this report, we show that ectopic expression of XLerk, a Xenopus homologue of the murine Lerk-2 (ephrin-B1) transmembrane ligand, causes dissociation of Xenopus embryonic blastomeres by the mid-blastula transition. Moreover, a mutant that lacks the extracellular receptor binding domain can induce this phenotype. The carboxyl-terminal 19 amino acids of the cytoplasmic domain of XLerk are necessary but not sufficient to induce cellular dissociation. Basic fibroblast growth factor, but not activin, can rescue both the loss of cell adhesion and mesoderm induction in ectodermal explants expressing XLerk. Collectively, these results show that the cytoplasmic domain of XLerk has a signaling function that is important for cell adhesion, and fibroblast growth factor signaling modulates this function.

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