ESR1 and breast cancer: As shown in Fig. 1C, MTUS1 Affymetrix probeset intensities were significantly lower in a majority (83.3%) of triple negative (ER−/PR−/HER2−) carcinomas as compared to luminal (ER+) and HER2+ tumor subgroups (Table 1), which supports the conclusion that reduced levels of MTUS1 are associated with breast cancer aggressive subtypes.