Quantitative mRNA analysis of breast tumors represents a routinely applied example of precision oncology. Currently the National Comprehensive Cancer Network (NCCN) recommends quantitative mRNA profiling (e.g., 21-gene RT PCR or Oncotype Dx assay) for nearly all surgically resected lymph node (LN) negative hormone receptor (HR) positive, HER2 negative breast cancers in order to predict recurrence risk with endocrine therapy compared to chemotherapy followed by endocrine therapy after surgery. The incidence of synchronous breast cancers is low and evidence concerning distant recurrence risk is limited, but the risk of distant recurrence from one or the other of two primary breast cancers appears to be higher than the recurrence risk of the single largest of the two cancers. In this report, a woman with synchronous primary breast cancers is described. Oncotype Dx testing was done on each of her two cancers. By assuming that the recurrence risk from each with adjuvant endocrine therapy is an independent event, the recurrence likelihood from one or the other or both is calculated. I propose that this calculated value more accurately should predict the recurrence from one or the other or both tumors with endocrine therapy or chemotherapy followed by endocrine therapy compared with using only the higher of the two Oncotype Dx estimated risks.