Several clinical studies have investigated whether HER2 protein overexpression or gene amplification influences the benefit of endocrine therapy in early-stage breast cancer in the adjuvant setting (Table 4) (Borg et al. 1994, Carlomagno et al. 1996, Berry et al. 2000, De Placido et al. 2003, Love et al. 2003, Dowsett et al. 2008, Rasmussen et al. 2008). The gene discussed is ERBB2; the disease is breast carcinoma.