ERBB2 and breast cancer: Other therapeutics that exhibit an additive or synergistic effect when used with hdacis include differentiation agents such as retinoic acid (ra) and all-trans retinoic acid (atra) for the treatment of leukemic cells; imatinib, an inhibitor of the Bcr-Abl tyrosine kinase expressed by the Philadelphia chromosome in chronic myelogenous leukemia; the breast cancer drug trastuzumab, a monoclonal antibody against the her2/neu (ErbB2) receptor; and 17AAG, an inhibitor of the oncogenic protein chaperone Hsp90 10,30.