ERBB2 and invasive breast carcinoma: Oncoproteins and their truncated protein forms are implicated in tumor progression, metastasis, and drug resistance.1–3 Human epidermal growth factor receptor 2 (HER2, a.k.a. erbB2, Uniprot P04626) can be expressed as the full-length receptor or as truncated forms (t-erbB2s).1 Truncated HER2 oncoprotein forms arise from metalloprotease-mediated shedding yielding membrane bound or cytoplasmic carboxy-terminal fragments (CTFs),4 alternative initiation of translation5,6 or RNA splicing variants.7 Full-length HER2 is amplified in 15–20% of invasive breast cancers.