ERBB2 and breast cancer: The predictive value of the 21-gene RS for chemotherapy benefit in women with ER-positive, HER2-negative breast cancer has been validated in several prospective clinical trials, including the National Surgical Adjuvant Breast and Bowel Project B-20 trial, Southwest Oncology Group (SWOG)-8814 trial, and Trial Assigning Individualized Options for Treatment (TAILORx) [8,9,10,11].