PLAC1 was initially identified as a protein expressed specifically in the placenta and other cells derived from the trophoblast lineage during embryonic development, but has also been found to be expressed ectopically in a wide range of human malignancies, particularly breast cancers. PLAC1 is a membrane-associated protein that is thought to serve a receptor-like function modulating cell-cell or ligand receptor interactions unique to the maternal-placental interface. Decreased expression of PLAC1 is associated with decreased expression of cyclin D1 and reduced expression of AKT kinase, which, combined with the fact that PLAC1 is expressed on the surface of cancer cells, suggests that PLAC1 may be an effective candidate for immunotherapeutic treatments of cancer.