ESR1 and breast cancer: Gene ontology analysis revealed that the triple positive (ERBB2 + /ESR1 + /PGR+) spots express the receptor ERBB4 (a.k.a. HER4) and its cognate ligand EREG (a.k.a. epiregulin) in addition to ESR and PGR. This is a reasonable finding given that HER4 is correlated with ER and PR expression in breast cancer, and when all of these receptors are found together, the prognosis can be more favorable (reviewed in ref. 28).