ERBB2 and neoplasm: This can be best illustrated by the differential response of patients to treatment with the HER2 inhibitor Herceptin; while HER2 negative patients do not response to Herceptin, only fraction of HER2+ patients respond to this treatment suggesting that additional unrecognized heterogeneity must exist within these patients and that more sophisticated strategies must be employed to investigate tumor heterogeneity and to develop therapeutic regimens.