To achieve its proper antitumor effect, T-DM1 relies on strong HER2 expression on the tumor cells, which is routinely assessed by immunohistochemistry status (IHC3+) often combined with fluorescence in-situ hybridization (FISH) [HER2 gene copy number of six or more or a HER2/CEP17 ratio of 2.0 or greater]6. The gene discussed is ERBB2; the disease is neoplasm.