In breast cancer, EGFR or erbB2 are overexpressed between 20 and 50% of cases, and increased expression is associated with shortened disease-free and overall survival, pointing to involvement in growth regulation of the tumours (Slamon et al, 1987; 1990; Lacroix et al, 1989; Klijn et al, 1992). This evidence concerns the gene ERBB2 and breast cancer.