Taken together, these data suggest that immuno-PET with [89Zr]Zr-DFO-trastuzumab—or, alternatively, [89Zr]Zr-DFO-pertuzumab—could play an important role in identifying patients with endometrial cancer who are likely to respond to HER2-targeted therapies. The gene discussed is ERBB2; the disease is endometrial cancer.