ERBB2 and breast cancer: Moreover, the importance of HER3 in human cancers is not limited to HER2-driven breast cancers, and HER3 has also been shown to be involved in for example tumorigenicity of HER3-overexpressing prostate cancer xenografts in vivo, to maintain in vivo proliferation of a subset of ovarian cancers via an autocrine signaling loop, and to be involved in endocrine resistance of ER+ breast cancer cell lines [10], [11], [12].