Estrogen Receptor (ER) is a hormonal transcription factor that plays important roles in breast cancer. It functions primarily through binding to the regulatory regions of target genes containing the consensus ERE motifs. In order to identify ER target genes and re-define the ERE motifs we performed ChIP-Seq analysis of ER in MCF7 breast cancer cell line. Applying a novel computational algorithm named Hybrid Motif Sampler (HMS), specifically designed for TFBS motif discovery in ChIP-Seq data, we were able to detect an improved ERE motif and reveal intra-motif dependency especially in neighboring base pairs. MCF7 cells were grown in starving medium (RPMI with 5% FCS) for 3 days prior to the treatment with 10 nM β-estradiol or vehicle control for 45 minutes. ChIP was done using an anti-ER antibody in both the ethl-treated and the E2-treated cells. ChIP-Seq sample prep and sequencing were done following the manufacture's protocol using the Genome Analyzer (Illumina). The read files were analyzed using ethl-treated as control for E2-treated, leading to one final peak file.