Such transgenic mice, termed the Δ16HER2-LUC line, develop several asynchronous metastatic mammary tumors between the age of 8 and 32 weeks, expressing heterogeneous levels of constitutively active, stable HER2 homodimers compared with the MMTVhuHER2 transgenic model, which form spontaneous human wtHER2-overexpressing mammary tumors after age 28 weeks [25,41]. This evidence concerns the gene ERBB2 and breast cancer.