ESR1 and breast cancer: Several evidences implicate GATA3 in the activation of the mammary differentiation program: (1) in normal tissue, it is necessary for the luminal compartment formation [9]; (2) GATA3 expression in BC strongly correlates with estrogen receptor (ER) expression [10]; (3) GATA3 functions in a complex with FOXA1 and ER to enhance transcription of ER-responsive genes [11]; and (4) ectopic expression in GATA3-negative basal-like BC cells is sufficient to induce luminal differentiation and inhibit tumor dissemination [12].