In 2006, the National Cancer Institute (NCI) launched the TAILORx trial to determine whether gene expression patterns in early-stage breast cancer can distinguish between women who are at high risk of cancer recurrence and, therefore, most likely benefit from adjuvant chemotherapy and women who have a low risk of recurrence and, thus, can be spared the toxic side effects of chemotherapy.

TAILORx, a phase 3 clinical trial, opened in 2006 and was designed to provide an evidence-based answer to the question of whether hormone therapy alone is not inferior to hormone therapy plus chemotherapy. The trial used a molecular test (Oncotype DX Breast Recurrence Score) that assesses the expression of 21 genes associated with breast cancer recurrence to assign women with early-stage, HR-positive, HER2-negative, axillary lymph node­–negative breast cancer to the most appropriate and effective post-operative treatment. The trial enrolled 10,273 women with this type of breast cancer at 1,182 sites in the United States, Australia, Canada, Ireland, New Zealand, and Peru.