Risk-stratification models such as the IHC4 score [13] and the Magee equationsTM [49,55,56,65,66,67,68], both of which use semi-quantitative information from the immunohistochemical assessment of ER, PR, HER2, and Ki-67 (four of the genes measured in the ODX panel), have been validated for identifying patients at low, moderate, or high risk of breast cancer relapse following endocrine therapy [49,67,68,69]. This evidence concerns the gene ESR1 and breast cancer.