The important role played by the sex hormone estrogen in disease and physiological processes has been well documented. However, the mechanisms by which this hormone elicits many of its normal as well as pathological effects are unclear. To identify both known and unknown genes that are regulated by or associated with estrogen action, we performed serial analysis of gene expression on estrogen-responsive breast cancer cells after exposure to this hormone. We examined approximately 190,000 mRNA transcripts and monitored the expression behavior of 12,550 genes. Expression levels for the vast majority of those transcripts were observed to remain constant upon 17b estradiol (E2) treatment. Only approximately 0.4% of the genes showed an increase in expression of >3-fold by 3 h post-E2 treatment. We cloned five novel genes (E2IG1-5), which were observed up-regulated by the hormonal treatment. Of these the most highly induced transcript, E2IG1, appears to be a novel member of the family of small heat shock proteins. The E2IG4 gene is a new member of the large family of leucine-rich repeat-containing proteins. On the basis of architectural and domain homology, this gene appears to be a good candidate for secretion in the extracellular environment and, therefore, may play a role in breast tissue remodeling and/or epithelium-stroma interactions. Several interesting genes with a potential role in the regulation of cell cycle progression were also identified to increase in expression, including Pescadillo and chaperonin CCT2. Two putative paracrine/autocrine factors of potential importance in the regulation of the growth of breast cancer cells were identified to be highly up-regulated by E2: stanniocalcin 2, a calcium/phosphate homeostatic hormone; and inhibin-b B, a TGF-b-like factor. Interestingly, we also determined that E2IG1 and stanniocalcin 2 were exclusively overexpressed in estrogen-receptor-positive breast cancer lines, and thus they have the potential to serve as breast cancer biomarkers. This data provides a comprehensive view of the changes induced by E2 on the transcriptional program of human E2responsive cells, and it also identifies novel and previously unsuspected gene targets whose expression is affected by this hormone.