Classification of breast tumours by their genomic profiles through cluster analysis identified four distinct subgroups. (a) Cluster membership outcomes are visualised in terms of BRCA abnormalities through principal component analysis (PCA). Two of the four identified subgroups were enriched for either BRCA1 or BRCA2 abnormalities referred to as the BRCA1-related (n = 11) and BRCA2-related subgroups (n = 9), respectively. The characters represent cluster memberships of each tumour with BRCA1 and BRCA2 abnormalities indicated, see bottom of the figure. It can be hypothesised here that component three reflects differences between BRCA1- and BRCA2-related tumours whereas component two reflects their similarities, see further in Additional data file 3. (b) The distribution of genomic instability indices (GII) differed considerably between the identified subgroups in that one of the subgroups displayed low genomic instability whereas the other three displayed high instability levels.