The ideal target antigen should be: (a) highly expressed with limited heterogeneity across the tumour and with low normal tissue expression, for example, T-DM1 targets HER-2, a receptor that reaches levels of expression of 2 × 106 on HER-2-positive cancer cells compared with 2 × 104 on other cells (Shefet-Carasso and Benhar, 2015); (b) there should be minimal antigen shedding to prevent the antibody binding to its target within the circulation; and (c) the antibody should be well internalised by receptor-mediated endocytosis and should not be modulated during endocytosis. The gene discussed is ERBB2; the disease is cancer.