ERBB2 and neoplasm: Although the CHO cell studies described above allow highly controlled structure-function studies to be performed in isolation from confounding variables, the HCC1569 in vivo studies allow the testing of these conclusions in an actual HER2-amplified tumor, accounting for all the complexities of cancer cell signaling, as well as any contributions from the in vivo microenvironment, including the potential role of in vivo ligands.