Multigene assays have been developed and validated to determine the prognosis of 
breast cancer. In this study, we assessed the additional predictive value of the 
70-gene MammaPrint signature for chemotherapy (CT) benefit in addition to 
endocrine therapy (ET) from pooled study series. For 541 patients who received 
either ET (n = 315) or ET + CT (n = 226), breast cancer-specific survival (BCSS) 
and distant disease-free survival (DDFS) at 5 years were assessed separately for 
the 70-gene high and low risk groups. The 70-gene signature classified 252 
patients (47%) as low risk and 289 (53%) as high risk. Within the 70-gene low 
risk group, BCSS was 97% for the ET group and 99% for the ET + CT group at 5 
years with a non-significant univariate hazard ratio (HR) of 0.58 (95% CI 
0.07-4.98; P = 0.62). In the 70-gene high risk group, BCSS was 81% (ET group) 
and 94% (ET + CT group) at 5 years with a significant HR of 0.21 (95% CI 
0.07-0.59; P < 0.01). DDFS was 93% (ET) versus 99% (ET + CT), respectively, in 
the 70-gene low risk group, HR 0.26 (95% CI 0.03-2.02; P = 0.20). In the high 
risk group DDFS was 76 versus 88%, HR of 0.35 (95% CI 0.17-0.71; P < 0.01). 
Results were similar in multivariate analysis, showing significant survival 
benefit by adding CT in the 70-gene high risk group. A significant and 
clinically meaningful benefit was observed by adding chemotherapy to endocrine 
treatment in 70-gene high risk patients. This benefit was not significant in low 
risk patients, who were at such low risk for recurrence and cancer-related 
death, that adding CT does not appear to be clinically meaningful.