The HER2/neu gene is one of the family of genes encoding transmembrane receptors for four growth factors, including the epidermal growth factor receptor (EGFR), HER2/neu, HER3, and HER4. The HER2/neu gene is localized on the long arm (q21) of chromosome 17. This gene codes for a 185 kDa receptor-type tyrosine protein kinase (p185 neu or c-erbB-2) similar to EGFR. This 1255 amino acid transmembrane glycoprotein is composed of three domains: an extracellular factor-binding domain, a transmembrane domain, and an intracellular domain with tyrosine kinase activity. The intracellular domain of HER2/neu has tyrosine kinase activity, which regulates cell growth and proliferation. HER2/neu is over-expressed in 20-25% of invasive breast cancers and is associated with an aggressive tumor, early relapse, and reduced survival.