Third, the receptors are frequently co-expressed in human breast cancer (Gasparini et al, 1994; Naidu et al, 1998; Witton et al, 2003); Naidu et al (1998), in a 220 patient study, showed that greater than 50% of all tumours expressed moderate to high levels of either ErbB2 or ErbB3, with their co-expression being higher than any other pair of ErbB family members. This evidence concerns the gene ERBB3 and breast cancer.