Only a few tumours displayed clonal intermingling throughout the whole tumour tissue.40 In a similar study, when multifocal breast cancer lesions with comparable tumour grades and ER and HER2 status were subjected to whole genome and targeted sequencing,41 genetically similar lesions were topographically located closer to each other than genetically distinct lesions, indicating a process of clonal evolutionary outgrowth. This evidence concerns the gene ERBB2 and breast cancer.