ERBB2 and breast cancer: Several biomarkers and clinical variables have been investigated, including serum HER2 extracellular domain [4, 5], p95HER2 truncated form of HER2 [6, 7] activation of the PI3K/AKT pathway [8], EGFR gene copy number [9], intrinsic breast cancer subtypes [10] and time to progression to first line trastuzumab [11], but none have found application in clinical practice.