- Background: The 21-gene recurrence score (RS) assay has been validated in retrospective studies as prognostic of distant disease recurrence and predictive of the benefit of adjuvant chemotherapy in estrogen-receptor-positive breast cancer (BC). There is limited published data on the impact of the assay on clinical practice, particularly in the context of a single practice with a unified treatment policy. Patients and methods: Between 2006 and 2009, RS was obtained on 135 patients in a single practice with a uniform treatment policy. Treatment recommendations before and after RS result were analyzed retrospectively. PreRS treatment recommendations were based on clinicopathological features and Adjuvant! Online (AO) calculated survival advantage. RS and AO survival advantage for adding chemotherapy were compared for each patient. Results: The distribution by risk group of RS was low— 49.6%, intermediate—37.8%, and high—12.6%. In 34 patients (25.2%, 95% confidence interval 17.9% to 32.5%), recommendation for chemotherapy was changed after obtaining assay result. Most changes (70.6%) were from chemotherapy to no chemotherapy. The RS correlated poorly with AO predictions. Conclusion: The 21-gene assay, when applied in a consistent manner in early-stage BC, changes treatment recommendations in one-quarter of patients tested.