The truncated ErbB2 receptor (p95ErbB2) is shown to differ from the full-length ErbB2 receptor in its association with other ErbB receptors. The truncated receptor preferentially associated with ErbB3, whereas full length ErbB2 heterodimerizes with either EGFR or ErbB3. Consistent with p95ErbB2 heterodimerization with ErbB3, it is shown that heregulin (an ErbB3 ligand) stimulates p95ErbB2 phosphorylation in breast cancer cell lines. Described herein are methods of identifying patients suitable for treatment with a p95ErbB2 inhibitor, and methods of treating such patients.