ERBB2 and breast cancer: Using in situ hybridization (ISH) techniques to identify HER2 gene amplifications, three distinct patterns of HER2 heterogeneity in human BC tumors have been described: i) “clustered type” with regions of HER2 amplified and non-HER2 amplified cells, ii) “mosaic type” with interspersed HER2 amplified and non-HER2 amplified cells, and iii) “scattered type” with isolated HER2-amplified cells in a tumor composed mainly of non-HER2 amplified cells.24