ER (Estrogen receptor / ESR1) is a biomarker that is prognostic for breast cancer and predicts response to hormonal therapy. Estrogen receptor is a nuclear hormone receptor with nuclear staining in malignant breast cancer cells, and usually has higher levels of expression in Luminal A subtypes compared to others (Jeselsohn, 2015). Levels of expression of estrogen receptor are used in conjunction with progesterone receptor (PR), HER2 and Ki-67 expression levels to subtype breast cancers for different downstream treatments. Most breast cancers have positive expression of ER protein, primarily in luminal tissues where the luminal A subtype sees high levels of expression. Staining for this target is expected to be nuclear. References: Nature reviews. Clinical oncology 2015 Jan; 12.1; 573-583, PMID: 26122181; World Journal of Clinical Oncology 2014 Aug; 5.3; 412-424, PMID:25114856;