To address this question, we sought i) to define the clinico-pathologic characteristics of HER2 heterogeneous breast cancers, ii) to determine the somatic gene copy number alterations in the HER2-positive and HER2-negative components of HER2 heterogeneous breast cancers, iii) to define the repertoire of somatic mutations in the HER2-positive and HER2-negative components of HER2 heterogeneous breast cancers, and iv) to identify potential driver genetic alterations of breast cancer based on the analysis of the HER2-negative components of HER2 heterogeneous breast cancers. The gene discussed is ERBB2; the disease is breast carcinoma.