Carcinoma-associated fibroblasts (CAFs) are associated with tumor progression and metastasis, and are able to activate estrogen receptor (ER) in breast cancer. We established a stable transformant of a human breast cancer cell line to detect CAF-specific ER-activating ability, and found that this CAF ability varied among tumors. Some studies have reported a high frequency of alterations among tumor suppressor genes in stromal cells, but do not generally agree as to the frequency. Moreover, the activation mechanism of CAF-induced estrogen signals, including the effects of these gene aberrations, is not fully understood. We investigated the relevance of tumor suppressor gene aberrations and ER-activating ability in CAFs derived from 20 breast cancer patients. Although CAF-specific ER-activating abilities varied among individual cases, all CAFs maintained wild-type alleles for 753 and PTEN. Also, copy number aberrations in these genes were not observed in any CAFs. Our results suggest that the ER-activating ability of the CAFs is regulated independently of aberrations in these genes: and that other mechanisms of tumorstromal interaction may affect activation of estrogen signals in breast cancer. (C) 2012 Elsevier Inc. All rights reserved.