We analyzed gene expression of MBD1, MBD2, MBD3, MBD4, and MeCP2 and protein expression of MBD1, MBD2, and MeCP2 in prostate cancer cell lines, benign prostate epithelium (BPH-1) cell line, 49 BPH tissues, and 46 prostate cancer tissues. The results of this study demonstrate that MBD2 gene is expressed in all samples and MeCP2 gene is expressed in all cancer cell lines but not in BPH-1 cell line. However, there was no protein expression for MBD2 and MeCP2 in cancer cell lines and cancer tissues. For CXXC sequence containing MBD 1, both protein and mRNA were expressed in cancer cell lines, cancer tissues, BPH-1 cell line, and BPH tissues. We observed that, in. BPH tissues and low-grade cancer tissues, MBD I protein expression was very high and gradually decreased with increase of cancer grade. Treatment of cancer cell lines with proteasome inhibitor (MG-132) did not restore expression of MBD2 and MeCP2 proteins. When prostate cancer cell lines were treated with hypontethylating agent, 5-aza-2'-deoxycytidine (DNMT inhibitor), HDAC I and HDAC2 expression was decreased. This is the first report demonstrating that CXXC sequence containing MBD1 is overexpressed and can be the major factor of hypermethylated chromatin segments through HDAC1/2 translocation and histone deacetylation in human prostate cancer. (C) 2003 Elsevier Science (USA). All rights reserved.