Methods for inhibiting cancer cell proliferation

A method of treating breast cancer in a subject, comprising delivering to a subject in need of such treatment a nucleic acid sequence encoding a mutant ErbB-2 polypeptide in an amount effective to inhibit cancer cell proliferation, wherein the mutant lacks a functional nuclear localization signal, cannot translocate to the nucleus of the cell in which it is present, and functions as a dominant-negative inhibitor of endogenous ErbB-2 by inhibiting nuclear translocation of endogenous ErbB-2 in the cell in which the mutant is present, wherein the cancer overexpresses ErbB-2, and the mutant ErbB-2 polypeptide retains intrinsic tyrosine kinase activity and does not inhibit endogenous ErbB-2 tyrosine kinase activity.