Comparison of an estrogen receptor related protein, the ERD5 antigen, with estrogen and progesterone receptors in breast cancer patients.

Eighty five breast cancer cytosols were assessed for ERD5-antigen (an estrogen receptor associated protein) using the Amersham immunoradiometric assay. Estrogen and progesterone receptors were measured using charcoal treatment to separate receptor-bound tritiated ligand from the excess free, or weakly-bound ligand. Patients whose cytosols were receptor-rich (ER+ PR+) were more likely to be ERD5-antigen positive and had higher quantitative levels of this protein, than patients whose cytosols contained no estrogen receptor protein. However, twenty nine percent of ER-negative cytosols showed significant ERD5 antigen levels. In this preliminary study ERD5 antigen levels showed no potential value in discriminating between breast cancer cytosols from ER positive and ER negative patients.