The transcription factor Forkhead box A1 (FOXA1), also known as hepatocyte nuclear factor 3 alpha (HNF3a), is involved in the embryonic development of several organ systems that are derived from the endoderm. Certain breast malignancies, normal breast ductal epithelium, and epithelial tissue in other organs exhibit anti-FOXA1 reactivity. The Anti-FOXA1 immunohistochemical reagent is helpful for sub-classifying breast carcinoma.