Immunohistochemical profile of ephrin A4 expression in human osteosarcoma. Ephrin receptors and ephrin ligands constitute one of the largest groups of tyrosine kinases. The division of ephrin receptors into type A or type B is determined by their ligand-binding specificities. Ephrin A4 as a ligand has a broad capacity to bind and stimulate different subtypes of ephrin A receptors. Little is known about the role of ephrins generally and ephrin A4 particularly in osteosarcoma. Ephrin A4 was immunohistochemically assessed on archival material from 46 primary osteosarcoma cases, 10 metastatic pulmonary lesions and 20 normal control bone specimens. Ephrin A4 was expressed in 100% of normal bone specimens, in 84.4% of primary osteosarcoma cases and in all metastatic pulmonary lesions. Cytoplasmic and nucleocytoplasmic patterns of ephrin A4 immunoreactivity were observed, with the predominance of the latter pattern in normal bone (100%), and in 43.5% of primary osteosarcoma cases, which showed a higher intensity of expression compared with normal bone (p<0.05). The cytoplasmic pattern is the only staining pattern seen in metastatic cases, which may suggest its role in enhancement of invasion and metastasis. The differences in the distribution of the two patterns of ephrin A4 may indicate a different biological activity of this molecule depending on its localization. The nuclear localization of ephrin A4 requires further investigation to clarify the mechanism and the significance of the nuclear trafficking of ephrin A4.