ESR1 and breast cancer: A further study revealed that high level of SIX1 protein was significantly associated with HER2+ status.[14] About 67.2% of HER2+ breast tissues were SIX1 strongly positive, while only 49.4% of HER2− tumor tissues were with strong staining of SIX1.[14] We assumed that high SIX1 mRNA level of HER2-overexpressing compensated the low SIX1 mRNA of basal-like breast cancer, contributing to the negative correlation between SIX1 mRNA and ER status at general level.