ERBB2 and breast cancer: With an estimated 2.3 million new cases in 2020, breast cancer has become the most commonly diagnosed cancer worldwide.1 Neoadjuvant systemic treatment (NAST) is routinely applied to women with locally advanced and/or HER2-positive or triple-negative (TNBC) breast cancer.2 Breast cancer patients with residual disease after NAST have a worse prognosis compared with those achieving a pathologic complete response (pCR).3 For non-pCR patients with TNBC or HER2-positive disease, post-neoadjuvant treatment with capecitabine or trastuzumab emtansine (T-DM1) showed improved survival.4,5