The correct interpretation of extracellular signals is essential for proper developmental and proliferative responses. Failure may lead to malignant growth. Receptors of the tyrosine kinase family play pivotal roles in these processes. In breast cancer the ErbB receptor family, in particular ErbB2 has an important role in ~20% of tumors. Other receptors, including members of the FGF receptor family and the Ret receptor are also aberrantly expressed in breast cancer. We are studying these receptors to understand how they contribute to cancer development using in vitro and in vivo tumor models.