Differential display was used to compare patterns of gene expression in two estrogen receptor (ER)-positive breast carcinoma cell lines (MCF7 and T-47D) and two ER-negative breast carcinoma cell lines (MDA-MB-231 and HBL-100). A 377-bp fragment was identified that was overexpressed in the ER-negative cell lines. Sequence analysis of this clone and comparison with the GenBank/EMBL databases indicated that it did not match any genes published previously. The expression pattern of this gene was inversely correlated with the expression of ER and has been termed ICERE-1 (inversely correlated with estrogen receptor expression). A longer clone of ICERE-1 was isolated from a MDA-MB-231 cDNA library and sequence analysis indicated that this 2168-bp cDNA contained an ORF encoding a protein of 234 amino acids that bears little similarity with any previously described protein sequence. Northern blot analysis of a panel of breast cancer cell lines demonstrated that an ICERE-1 mRNA of approximately 2.2 kb was abundantly expressed in the ER-negative breast carcinoma cell lines, MDA-MB-231 and HBL-100, and the ER-negative cell lines. HEC-1-B, HeLa, and 293. Expression of ICERE-1 was absent or minimal in the ER-positive breast carcinoma cell lines MCF7, T-47D, MDA-MB-361, ZR-75-1, BT-474 and BT-20. Reverse transcription/PCR was used to examine ICERE-1 expression in 29 primary breast carcinomas, 15 of which had been designated as ER positive and 14 as ER negative by immunohistochemistry. The expression level of ICERE-1 was significantly lower (P < 0.001) in the ER-positive tumors compared with the ER-negative tumors. The pattern of expression of ICERE-1 indicates that this gene may be involved in tumor biology specific to hormonally unresponsive breast cancers.