Patients with HER2-positive (HER2+) breast cancer have a poor prognosis, including a greater risk of relapse and shortened survival, compared with patients who have HER2-negative (HER2–) breast cancer (Slamon et al. 1987; Toikkanen et al. 1992; Andrulis et al. 1998; Pauletti et al. 2000; Rubin and Yarden 2001). This evidence concerns the gene ERBB2 and breast cancer.