A classic example is ITH of oestrogen receptors, which may be positive in just 1% of cells; in fact, this ITH has been reported to be associated with long‐term risk of fatal breast cancer.106 Other examples are the heterogeneous expression of cyclin D1 in mantle cell lymphomas, which otherwise carry a homogeneous CCND1–IGH translocation107 and the subtype of HER2+ breast or gastric tumours that can range from 10% to 100% positive cells.9, 108. This evidence concerns the gene CCND1 and breast cancer.