Human estrogen receptor (ER antibody) is a 66 kDa protein that acts as an estrogen-dependent, nuclear hormone receptor. Studies have shown ER is present in the nuclei of epithelial cells in normal breast and endometrial tissues, as well as a subset of breast carcinomas. The SP1 clone is a high affinity rabbit monoclonal antibody directed against an epitope of the C-terminus of the ER protein. Studies have shown that the SP1 clone has higher sensitivity than available mouse monoclonals in breast cancer. In some instances, SP1 staining can be obtained even without antigen retrieval.