ERBB3 and prostate cancer: Some studies have shown that the HER3 ligand, heregulin, is the main cause of drug resistance against PI3K inhibitor GDC-0941 by allowing prostate cancer cells to fend off the signaling pathway; while targeting HER2 and HER3 with corresponding antibodies restored the sensitivity of GDC-0941 and allowed the GDC-0941 to again stop the growth of the cancer.