ERBB2 and breast cancer: Mammary tumors also arose more rapidly (Figure 4E) and with greater multiplicity (Figure 4—figure supplement 4) in WAP-tva mice than in MMTV-tva mice, consistent with previous evidence of alveolar or alveolar progenitor cells as the preferred cell of origin for ErbB2-initiated mammary tumors (Li et al., 2003; Andrechek et al., 2004; Henry et al., 2004; Jeselsohn et al., 2010).