By means of protein expression profile, mass spectral and/or RT-PCR analyses we found for the first time IgG Fc binding protein (Fc gammaBP), distinct from Fc gamma receptors is expressed in both human and mouse prostates. There is a strong correlation between downregulation of Fc gammaBP and the progression of prostate cancer in Transgenic Adenocarcinoma Mouse Prostate (TRAMP) mice. Fc gammaBP gene expression is significantly reduced or lost in human prostate tumor tissues and prostate cell lines. The differentially expressed Fc gammaBP could reflect its potential role in prostate malignancy as well as an indicator for progression of the cancer.