ERBB2 and breast cancer: Use of the 21-gene RS has been associated with a significant decline in adjuvant chemotherapy use in early-stage HR-positive, ERBB2-negative breast cancer in the US, with many patients appropriately spared the added toxic effects of cytotoxic chemotherapy when found to have a genomically low risk of breast cancer recurrence.24 Clinical controversy remains,23 however, regarding the use of genomic assays to guide treatment recommendation across all populations, such as for younger patients and racial and ethnic minority patients.