Overexpression of HER2 (human epidermal growth factor receptor 2) occurs in around 20% of breast cancers. This particular subtype is known to be very aggressive and had the worst prognosis prior to the introduction of targeted therapy. The development of anti-HER2 therapies such as trastuzumab and pertuzumab has shown to dramatically improve the prognosis of patients with HER2-positive breast cancer.