Oncomodulin is a 12-14kD member of the parvalbumin family of Ca++binding proteins. It is expressed in early embryonic cells, placenta, and in tumors. OM was originally thought to have expression restricted to neoplastic tissues, early embryonic cells and certain tumor cell lines. Recent research shows that oncomodulin is also expressed and secreted by macrophages where, in the retina, it binds to retinal ganglion cells (RGCs) and functions to promote axon regenerationin early embryonic cells, placenta, and in tumors. OM is both cytoplasmic, and secreted. Rat OM is 109aa in length. It contains a vestigial Ca++binding site 733aa and two EF hand domains, the latter of which contains one highaffinity Ca++binding site 78108aa. Relative to parvalbumin alpha, OM has a lower pI (<4.8), a higher affinity for Ca++, and they share only 50aa identity. Full length rat OM shares 95-89aa identity with mouse and human OM, respectively.