In contrast to our in vitro findings, C4HD-TAM-67-hErbB-2ΔNLS tumors showed the lowest proliferative rates among our preclinical models, suggesting that nuclear ErbB-2 modulates genes involved in in vivo breast cancer proliferation, which do not play a key role in in vitro proliferation, independently of the assembly of the AP-1/Stat3/ErbB-2/PR complex. The gene discussed is STAT3; the disease is breast cancer.