ERBB2 and breast cancer: Results from the DESTINY-Breast06 trial demonstrated that not only patients with HER2-low breast cancer but also those with HER2-ultralow disease (defined as IHC 0 with incomplete and faint staining in ≤ 10% of tumor cells) benefited from T-DXd, suggesting that identifying HER2-ultralow disease is also clinically relevant [7–9].