ERBB2 and neoplasm: Hein et al. [44] observed that oncogenic transformation of the luminal lineage resulted in a small percentage of tumor cells co-expressing Krt5; however, their model utilized HA-tagged Polyoma Middle T antigen (PyMT) and Erb-B2 oncogenes to both induce transformation and label the luminal lineage, whereas our lineage-tracing strategy uncoupled the process of oncogenesis from lineage labeling.