ERs (ERalpha and ERbeta) mediate cellular signaling of estrogens, belonging to the nuclear receptor family of transcription factors. The mechanism of regulatory actions of ERs is that ligand-bound ERs bind directly to estrogen response elements (ERE), in the promoters of target genes or via protein-protein interactions with other transcription factors. The dysfunction of ERs has been demonstrated to be closely related with breast cancer, prostate cancer and lung cancer. Signosis has developed ER filter assay for monitoring DNA binding activity of ER. Unlike conventional gel shift assays, ER filter assay is a high throughput assay and it can analyze multiple samples simultaneously. Meanwhile, the assay can be set up for a few samples with less tedious than non-radioactive gel shift assay or much safe than 32P based gel shift assay.