Tumours exhibiting mutations within RTK pathway constituents (e.g. RAS, RAF, PIK3CA, PTEN, AKT, mTOR) or their upstream receptors have shown altered sensitivity to many different pathway targeted inhibitors including trastuzumab (HerceptinTM) in breast cancer, gefitinib (IressaTM) in non small cell lung cancer (NSCLC), cetuximab (ErbituxTM) in colorectal cancer (CRC) and vemurafenib (ZelborafTM) in skin melanoma when compared to their wild type counterparts. The gene discussed is PIK3CA; the disease is breast carcinoma.