ERBB2 and breast cancer: Erb-b2 receptor tyrosine kinase 2 (ERBB2; formerly HER2/neu)–positive breast cancer was first defined as breast cancer with an amplification of ERBB2 leading to ERBB2 overexpression.1 ERBB2 is clinically assessed with immunohistochemistry (IHC), with expression scored from 0 to 3+, and/or with in situ hybridization (ISH), which can detect ERBB2 amplification.