Estrogens play crucial roles in regulating gene expression in physiological and disease states. Estrogens acts through estrogen receptors (ERs) and their binding sites in genomic DNA to modulate transcription by RNA polymerase II. Although recent gene-specific and genomic analyses have provided considerable information about of estrogen-dependent transcription, many aspects of the estrogen signaling network have not yet been elucidated. The goal of my studies was to uncover new information about the immediate and direct effects of estrogen signaling at the cell membrane, in the cytoplasm, and in the nucleus to elucidate the underlying regulatory networks. First, I investigated an ER transcriptional coregulators, SWI/SNF, an ATPdependent chromatin remodeling complex. I explored the molecular functions of the BAF57 and BAF180 subunits of SWI/SNF using a quantitative proteomic approach called SILAC (Stable Isotope Labeling by Amino Acids in Cell Culture). I found that depletion of BAF57 results in a significant depletion of BAF180 from the SWI/SNF complex without decreasing the total cellular BAF180 levels, resulting in an accumulation of cells in the G2/M phase. Knockdown of BAF57 also causes transcriptional misregulation of cell cycle-related genes involved in the late G2 checkpoint. Collectively, these studies have elucidated the role of BAF57 and BAF180 in the transcriptional control of cell proliferation. Second, I have used GRO-Seq (Global Nuclear Run-On and Massively Parallel Sequencing) to explore the immediate effects of estrogen signaling on the transcriptome of breast cancer cells. I found that estrogen directly regulates a strikingly large fraction of the transcriptome in a rapid, robust, and unexpectedly transient manner. In addition to protein coding genes, estrogen regulates the distribution and activity of all three RNA polymerases, and virtually every class of non-coding RNA that has been described to date. I also identified a large number of previously undetected estrogen-regulated intergenic transcripts, many of which are found proximal to ER[alpha] binding sites. These results provide the most comprehensive measurement of the primary and immediate estrogen effects to date. I expect that genome-wide inferences based on the direct estrogen-regulated transcriptome in combination with estrogen-regulated signaling pathway will be useful for understanding estrogen biology.