There exist several examples of therapies which have only been approved for a specific subset of patients identified by a predictive biomarker, such as human epidermal growth factor receptor 2 (HER2) overexpression for the treatment of breast cancer patients, Kirsten rat sarcoma viral oncogene homolog (KRAS) mutations in patients with colorectal cancer, and epidermal growth factor receptor (EGFR) mutations for patients with non-small cell lung cancer [1]. This evidence concerns the gene ERBB2 and breast cancer.