ERBB2 and breast cancer: To identify novel contributing factors to breast cancer development and progression, we performed an insertional mutagenesis screen on primary mouse mammary epithelial cells (MMECs) that are partially transformed by expression of the neu proto-oncogene under the control of the mouse mammary tumor virus promoter.5 At 7 weeks of age, these mice show hyperplasia of their ductal trees indicative of neu overexpression.