In HR-positive/HER2-negative primary breast cancer cohort, a statistically significant association was observed between HER2-low expression and both Luminal B-like phenotype (Luminal B-like proportion in HER2-0 vs HER2-low: 51.9% vs 64.7%, respectively, p = 0.023) and higher ER expression (mean ER expression in HER2-0 vs HER2-low: 71.2% vs 76.8%, p = 0.032). This evidence concerns the gene ESR1 and breast cancer.