Accordingly, RS testing has facilitated the personalization of cancer treatment for those with early-stage ER+/HER2− disease, leading to the endorsement of the 21-gene expression assay by several expert panels and oncology societies in their recommended guidelines for early-stage breast cancer, including the American Society for Clinical Oncology (ASCO), the National Comprehensive Cancer Network (NCCN), and the European Society of Medical Oncology (ESMO), among others [5,6,7,8,9]. This evidence concerns the gene ESR1 and cancer.