The ZEPHIR trial has shown that immuno-PET with 89Zr-trastuzumab effectively delineated patients with breast cancer who would likely respond to a HER2-targeted antibody–drug conjugate (i.e., trastuzumab–emtansine), and our own work has demonstrated that HER2-targeted immuno-PET can identify both HER2-positive and HER2-low breast cancer, even in patients with HER2-negative primary disease (28). Here, ERBB2 is linked to breast cancer.