Estrogen has been closely associated with the genesis and malignant progression of breast cancer. However, the molecular mechanism underlying the effects of estrogen is far from being completely clarified. We previously developed a custom-made cDNA microarray consisting of approximately 200 estrogen-responsive genes in breast cancer cells. Using this system, we found one estrogen-induced gene in various cancer cell lines, including breast cancer MCF-7 cells, which encode a zinc-finger transcription factor, EGR3 (early growth response 3). Northern blot analysis of estradiol-treated MCF-7 cells showed rapid and robust induction of Egr3, and addition of cycloheximide or ICI 182,780 suggested that Egr3 is the bona fide target for the estrogen receptor alpha (ERalpha). Using stable transformants derived from MCF-7 cells which were transfected with expression-controllable Egr3-expression vector, we demonstrated that Nab2 is one of the target genes for EGR3. Microarray analysis of the transformants revealed other candidate EGR3-induced genes. These strategies could be useful for analyzing downstream genes of ERalpha, and may contribute to elucidating the extensive signaling network of estrogen stimuli. Furthermore, a reporter assay using the upstream region of fasL probably involving escape from the immune system revealed that fasL is another target gene for EGR3. The roles of EGR3 in the physiology of breast cancer are discussed.