To illustrate the concepts of penetrance and coverage as applied to functional genomic screens in TCLs, we examined a well-characterised and therapeutically actionable oncogene addiction effect that operates in breast cancer, namely that associated with amplification of the epidermal growth factor receptor oncogene, ERBB2. Amplification and overexpression of ERBB2 is used clinically to stratify breast cancer patients for treatment with ERBB2-targeting agents such as the monoclonal antibody trastuzumab [30]. The gene discussed is ERBB2; the disease is breast cancer.