Be precise! The need to consider the mechanisms for CEP17 copy number changes in breast cancer.

A recent paper by Marchiò and colleagues indicated that true polysomy of chromosome 17 is rare and that coincident CEP 17 centromere amplification may account for its overestimation in FISH testing for HER2 amplification, with a treat/not treat decision as the final important readout. Here the importance of relating HER2 copy number to cell count rather than a ratio with CEP17 is emphasized. This is central to ensuring that all who may respond to potentially life-saving treatment with herceptin/lapatininib are not denied these drugs.