Among patients with HER2-negative primary breast cancer, the proportion of HER2 discordant cases was numerically higher in HR-positive/HER2-negative cohort (HER2 discordant cases in HR-positive/HER2-negative vs triple-negative: 45.5% vs 36.7%, respectively, p = 0.17) and this difference was mainly driven by the proportion of HR-positive/HER2-0 breast cancer cases switching to HER2-low expression. This evidence concerns the gene ERBB2 and breast cancer.