ESR1 and breast cancer: Furthermore, Gene-expression profiling analysis has been used to dissect the heterogeneity of BC into six subtypes: Luminal A (ER+, low grade), Luminal B (ER+; high grade), HER2 positive (HER2-amplification), basal-like (ER-; HR-; HER2-), normal-like and most recent “claudin low” subtypes [37–40].