Introduction
Estrogen receptor (ER)-α and ER-β are believed to mediate 		 the action of estradiol in target tissues [1,2]. These two receptors, which belong to the steroid/retinoic 		 acid/thyroid receptor superfamily [3], contain several 		 structural and functional domains [4] that are encoded by 		 two messenger RNAs that contain eight exons [5,6]. Upon ligand binding, ER-α and ER-β proteins 		 recognize specific estrogen-responsive elements located in DNA in the proximity 		 of target genes, and through interactions with several coactivators modulate 		 the transcription of these genes [7].
Several ER-α and ER-β variant messenger RNAs have been 		 identified in both normal and neoplastic human tissues [8,9,10,11,12]. Most of these variants contain 		 a deletion of one or more exons of the wild-type (WT)-ER messenger RNA. The 		 putative proteins encoded by these variant messenger RNAs would therefore be 		 missing some functional domains of the WT receptors and might interfere with WT 		 ER signaling pathways. Indeed, in vitro functional studies have shown 		 that some recombinant ER-α variant proteins can affect estrogen-regulated 		 gene transcription. For example, the variant protein encoded by exon 3 deleted 		 ER-α variant (ERD3) messenger RNA, which is missing the second zinc finger 		 of the DNA binding domain, has been shown [13] to have a 		 dominant negative activity on WT ER-α receptor action. A similar dominant 		 negative activity has been observed for ERD5 variant protein (encoded by an 		 ER-α variant messenger RNA deleted in exon 5 sequences), which is missing 		 a part of the hormone-binding domain of the WT molecule [14]. Interestingly, a constitutive hormone-independent 		 activity [15] and a WT enhancing activity [16] have also been attributed to ERD5 variant protein in 		 different systems. The relevance of the levels achieved in these transfection 		 experiments to in vivo expression remains unclear. It should also be 		 noted that these functional activities are likely to be cell-type and promoter 		 specific [8].
The discovery that these ER-α variants are expressed in both 		 normal and neoplastic human breast tissues, however, raised the question of 		 their possible role in breast tumorigenesis [8]. We have 		 previously reported an increased relative expression of ERD5 messenger RNA and 		 of ERC4 messenger RNA, another ER-α variant messenger RNA that is 		 truncated of all sequences following the exon 2 of the WT ER-α [17], in breast tumor samples versus independent normal breast 		 tissues [18,19]. In contrast, 		 Erenburg et al [20] recently reported a 		 decreased relative expression of ERD3 messenger RNA in tumor tissues and cancer 		 cell lines versus independent normal reduction mammoplasty samples. Those data, 		 which suggested that alteration in ERD5, ERD3 and clone 4 messenger RNA 		 expression might occur during breast tumorigenesis, were obtained in tissues 		 from different individuals, and possible interindividual differences cannot be 		 excluded.
In order to clarify this issue, we investigated the expression of 		 these three variant messenger RNAs in normal breast tissues and their matched 		 adjacent primary breast tumor tissues.