Within HER2 IHC 3 + BC patients, there was no statistically significant association between ER status and pCR rates, unlike BC patients with HER2 IHC 2 + /Amplified where negative ER status were significantly associated with increased pCR rates (P < 0.001) (Supplementary Fig. 2B and Supplementary Table 3). Here, ESR1 is linked to breast cancer.