Loss of expression of a 55 kDa nuclear protein (nmt55) in estrogen receptor-negative human breast cancer.

We have identified and characterized a 55 kDa nuclear protein (referred to as nmt55) from human breast tumors and MCF-7, human adenocarcinoma breast cell line, using site-directed monoclonal antibodies. Measurements of estrogen receptors (ER) and progesterone receptors (PR), by ligand binding assays, in cytosols of 63 human breast tumors permitted classifications of these tumors into four phenotypes (ER+/PR+, ER=/ PR-, ER-/PR-, ER-/PR+). Nuclear protein (nmt55) expression in these tumors, as determined from Western blot analyses, showed a statistically significant association (p = 0.001) with tumor hormonal phenotype. Review of the pathologic characteristics of tumors analyzed suggested that lack of nmt55 expression was significantly associated with mean tumor size (p