Recent studies have indicated that a complex machinery of transactivation of target genes by estrogen or antiestrogen through estrogen receptor (ER) exists. However, the substantial roles of ER-beta, coactivators, and corepressors in the development and progression of breast cancer remain to be elucidated. To obtain some clue to these roles, we screened the expression levels of ER-alpha, ER-beta, coactivators (SRC-1, TIF2, AIB1, CBP, and P/CAF) and corepressors (N-CoR and SMRT) in 6 normal mammary glands, 6 intraductal carcinomas, 22 invasive ductal carcinomas, and 7 breast cancer cell lines using a multiplex reverse transcription-PCR. ER-alpha mRNA expression levels significantly correlated with ER-alpha protein levels measured by enzyme immunoassay in the breast cancer tissues and cell lines. A significant correlation of expression levels was observed between ER-alpha and TIF2, AIB1, P/CAF, and N-CoR, and between ER-beta and AIB1 and CBP in the tissue samples. A significant correlation was also observed between ER-alpha and ER-beta and between ER-beta and CBP in the cell lines. The expression levels of ER-alpha, TIF2, and CBP were significantly higher in the intraductal carcinomas than those in the normal mammary glands. In addition, the expression levels of ER-alpha and N-CoR were significantly higher in the intraductal carcinomas than those in the invasive ductal carcinomas. These findings suggest a positive correlation of expression levels among ER-alpha and cofactors and among ER-beta and cofactors, an up-regulation of expression levels of ER-alpha and cofactors during the development of intraductal carcinomas from normal mammary glands, and a decrease in their expression levels during the progression of breast cancer.