OBJECTIVES: Tumour gene expression analysis is useful in predicting adjuvant chemotherapy benefit in breast cancer (BC). The 21-gene assay test is used to estimate the risk of recurrence and to predict the benefit of adjuvant chemotherapy (CT) at an early stage of hormone-receptor-positive (ER+) BC. This study aims to examine the implications of Recurrence Score (RS) testing on treatment decisions in France. METHODS: We conducted an open multicenter, cohort study, with voluntary participation, involving women with ER+, HER2-, BC. Performing the assay was left at the discretion of the oncologist. The final treatment decision was discussed among the tumor board. The therapeutic changes after knowledge of RS were collected. RS and classical BC prognostic markers were investigated in parallel with the budget impact of the assay. RESULTS: The study enrolled 149 patients in 38 sites. Mean age: 53.4 years, mean tumor size: 16.8 (SD 4.24). 75% of Patients had grade 2 tumor. The distribution of RS was 61.5% low, 33.1% intermediate and 5.4% high. When analyzed according to age, RS was low in 67.4% and 58.4% of patients aged < 50 and ≥ 50, respectively. RS was low in 65.6% of tumor ≤ 20 mm and 67.7% of tumor > 20 mm. Based on RS results, tumor board changed treatment decision for 76% of patients, avoiding CT for 75% of the population which was initially recommended for a CT and endocrine therapy. Patients with intermediate RS received CT in 47% and with low RS in 7.6%. The potential budget impact was estimated at -375 000€. CONCLUSIONS: Results of this “real life” study confirm that the 21-gene assay has substancial impact on treatment decision taken by tumor boards. These findings warrant further consideration for the use of this genomic assay in patients with early stage BC in France.