The ZEPHIR trial, in which patients with advanced HER2 positive breast cancer underwent HER2 PET/CT and [18F]FDG PET/CT before T-DM1 initiation, found 93/265 (35%) lesions were HER2-negative, and of these, 18 (19%) lesions (from 11 patients), responded anatomically after three T-DM1 cycles, resulting in an 81% NPV of the HER2 PET/CT. The gene discussed is ERBB2; the disease is breast cancer.