ESR1 and breast carcinoma: Based on the status of ER, PR, and HER2, breast cancers are grouped into 5 distinct molecular subtypes, namely luminal A, luminal B, HER2-overexpressing, basal-like, and normal-like.[2] Among these subtypes, luminal breast cancer accounted for the majority of breast cancer and tended to be with a better outcome, while patients with basal-like subtype have a poor survival rate.[2] In this study, we found that in contrast to high expression of SIX2 and SIX3, the level of SIX1 mRNA was significantly lower in basal-like tumors as compared to luminal subtype.