Therefore, we conducted a study in which patients with advanced HER2-positive breast cancer underwent both 89Zr-trastuzumab (HER2) PET/CT and FDG PET/CT before the first administration of T-DM1, followed by an early FDG metabolic response assessment performed just before the second treatment cycle (ZEPHIR study). The gene discussed is ERBB2; the disease is breast carcinoma.