ERBB2 and breast cancer: Harvested mammary epithelial cells from GLUT1fl/fl mice (Figures S2, S3) were transformed in culture with the polyomavirus middle T antigen, an oncogene that shares many similarities to ErbB2 and is frequently used in mouse models of breast cancer [35], to establish a stably proliferating cell line named “G1fP” (for Glut1 floxed PyVMT.