ERBB2 and breast cancer: Interestingly, β1-integrin and β4-integrin- heterodimers (reviewed in [2]) are both expressed by breast epithelial cells (mostly as α3β1 and α6β4) and have been shown to be required for tumorigenesis and metastasis in the MMTV/PyMT and MMTV/Neu (rat homolog of ErbB2) mouse models of breast cancer [3,4], respectively.