ERBB2 and cancer: This heterogeneity, far from being an unimportant epiphenomenon of disease development, has proven important in therapy: major advances in cancer therapeutics and patient outcomes have been achieved through the study and targeting of specific molecules and molecular mechanisms such as the estrogen receptor, and HER2/neu, aberrant expression of which occurs in approximately 30% and 25% of breast cancers, respectively [1], [2], [3].