In clinical specimens, the co-expression of ERα (ESR1) and GPER has been found in ∼40% of the primary breast cancer cases and GPER expression, as an independent unfavorable factor, has been found to correlate with relapse-free survival in patients treated with TAM (Filardo et al. 2006, Ignatov et al. 2011). This evidence concerns the gene ESR1 and breast cancer.