Ephrins and Eph receptors in stem cells and cancer.

Eph tyrosine kinase receptors and their ephrin ligands are expressed in most adult stem cell niches and in many types of tumors. They maintain tissue homeostasis by controlling the proliferation of stem and progenitor cells, although in divergent ways in different tissues. Eph receptors can also act as both tumor promoters and suppressors in different contexts. The recent characterization of the signaling pathways employed by Eph receptors has resulted in new suggestions for therapeutic strategies.