As stated above, the ErbB2 and ErbB3 receptors are frequently co-expressed in human breast cancer, but potentially more indicative of the importance of ErbB2/ErbB3 heterodimers in driving breast cancer progression is the link between escape from ErbB2-targeted therapies and a gain in ErbB3 activity or expression. This evidence concerns the gene ERBB2 and breast carcinoma.