Results of reverse-transcriptase polymerase chain reaction (RT-PCR) of 21 selected genes from the paraffin-embedded tumour blocks of female patients diagnosed with ER-positive, HER2-negative, LN-negative breast cancers from the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-20 trial were used to derive the algorithm which is used routinely within the multigene assay, which was then independently validated using data from the NSABP B-14 trial [10,12]. Here, ESR1 is linked to breast cancer.