ERBB2 and breast cancer: Hein and colleagues [39] used the LC-restricted avian retroviral vector and in vivo tracing experiments to show that the lineage-specific expression of ERBB2 or PyMT in committed LCs can induce these cells to generate mammary tumors derived from additional cellular lineages, revealing that either ERBB2 or PyMT induces the plasticity of LCs during tumorigenesis.