We have characterized a large set of HER2+ BC in comparison to HER2- BC using a combination of molecular techniques to delineate the HER2-amplicon in high-detail, and to pinpoint, on a genome-wide scale, critical regions of focal amplifications, gains and losses that may be important for tumor development and reflect the heterogeneity of HER2+ BC. This evidence concerns the gene ERBB2 and breast cancer.