ERBB2 and breast cancer: Given the above data, we decided to focus on the HER2+BC subtype and took advantage of the FVB Δ16HER2 transgenic mouse model, expressing the Δ16 variant of HER2 under the mouse mammary tumor virus (MMTV) promoter, in which females spontaneously develop very aggressive multifocal and invasive mammary carcinomas, with onset of palpable tumors at an average age of 15.28 weeks16–18.