ESR1 and breast cancer: Notably, the 21-gene Recurrence Score (RS) (Oncotype DX; Genomic Health, now part of Exact Sciences, Madison, WI, USA) assay has been validated to predict clinical response of ER-positive/HER2-negative breast cancer to neoadjuvant hormonal therapy using letrozole in the prospective TransNEOS study (n = 295 patients)91.