The cytoplasmic domain or intracellular domain (ICD) of the HER2/neu protein that exhibits tyrosine kinase activity. Based on sensitization theory, co-administration of trastuzumab (anti-HER-2/neu monoclonal antibody) and HER-2/neu intracellular domain protein may result in the potentiation of a HER2/neu-specific cytotoxic T lymphocyte (CTL) response against tumor cells overexpressing the HER2/neu protein. HER-2/neu protein, a glycoprotein cell surface receptor that is composed of an extracellular domain (ECD), a transmembrane domain, and an ICD, is overexpressed by many adenocarcinomas including breast adenocarcinoma.