GATA3 is a zinc-finger transcription factor involved in development and differentiation of T helper (Th)2 cells, and has diverse functions in immune regulation. It is expressed in urothelial and breast epithelial neoplasms, as well as salivary gland and parathyroid tumors. It can also be significantly mutated in luminal breast cancers, and it is highly expressed in breast carcinoma. It stains at high levels of expression across many subtypes including triple-negative breast cancers for both primary and metastatic carcinomas, and it is considered a biomarker for the Luminal A subtype of epithelial cells. Staining for this target is expected to be nuclear. References: Trends in Immunol 2014 35(6):233; Adv Anatomic Pathol 2013 20(5):352, Goncalves, 2014; Kandalaft, 2016; Sangoi, 2016; Yersal, 2014