ERBB2 and triple-negative breast carcinoma: Here, we provide direct evidence to demonstrate that, like triple-negative breast cancers [9], a subset of HER2-positive cancers are mosaics at diagnosis, and the subpopulations of cancer cells in these tumors may differ on the basis of the presence of HER2 gene amplification, their perceived main driver, in addition to the presence of subclonal mutations affecting other genes.