ERBB2 and breast cancer: Approximately 25–30% of patients with breast cancer overexpress HER2 and this overexpression is correlated with gene amplification and poor clinical outcome (King et al, 1985; Slamon et al, 1987, 1989; van de Vijver et al, 1988; Gusterson et al, 1992; Hynes and Stern, 1994).