Ephrin type-A receptor 1/EphA1 is a glycosylated member of the Eph family of transmembrane receptor tyrosine kinases. The signaling pathway downstream of the receptor is referred to as forward signaling while the signaling pathway downstream of the ephrin ligand is referred to as reverse signaling. The A and B classes of Eph proteins are distinguished by Ephrin ligand binding preference but have a common structural organization. Eph-Ephrin interactions are widely involved in the regulation of cell migration, tissue morphogenesis, and cancer progression. EphA1 can form pH sensitive cishomodimers on the cell surface. Membrane-bound or clustered Ephrin ligands interact with EphA1 and activate its kinase domain which is capable of Ser, Thr, and Tyr phosphorylation. Reverse signaling is propagated through the Ephrin ligand. EphA1 is widely expressed in differentiated epithelial cells, particularly in bone marrow, spleen, thymus, and testes. EphA1 is upregulated or downregulated in a variety of human carcinomas and is implicated in tumor invasiveness.