In the KRISTINE trial, a phase III trial of neoadjuvant T-DM1 and pertuzumab in HER2+ BC, a subgroup of 15 patients treated with T-DM1 and pertuzumab who had locoregional progression before surgery showed high heterogeneity in HER2 expression, which may have contributed to the worse clinical outcomes observed with T-DM1 treatment [45]. Here, ERBB2 is linked to breast cancer.