ERBB3 and breast cancer: Because MM-121 mainly inhibits activation of erbB3 and Akt in erbB2-overexpressing breast cancer cells, it is conceivable to hypothesize that MM-121 may abrogate erbB3 signaling-mediated therapeutic resistance to tamoxifen, trastuzumab, and other chemotherapeutic agents, such as doxorubicin.