ERBB2 and breast cancer: To examine the consequences of LKB1 loss on mammary tumor formation driven specifically by the ErbB2 oncogene, we crossed mice containing a floxed LKB1 gene [18] with a MMTV/NIC mouse model that expresses an activated form of ErbB2 (NDL2-5) and Cre recombinase from a single bicistronic transgene [19].