Of note, 70 - 80% of patients HER2 overexpressing breast cancer do not respond to trastuzumab due to either primary or acquired resistance. One of the important mechanisms for trastuzumab resistance is the accumulation of truncated forms of the HER2 receptor which lack the extracellular trastuzumab-binding domain (Figure 2). P95HER2, an amino terminally truncated carboxyl terminal fragments of HER2, is frequently found in HER2(+) breast cancer cell lines and tumor specimens (~20%)[11]. Intriguingly, recent study showed that p95HER2 (+) breast cancer cells were resistant to trastuzumab but remained sensitive to the antiproliferative effects of the tyrosine kinase inhibitor lapatinib, both in vitro and in vivo[11]. In addition, patients with p95HER2(+)breast cancer were resistant to trastuzumab with significantly lower response rate when compared with full-length HER2(+) breast cancer (11.1% vs 51.4%, respectively; P = 0.029).