The oncogene HER2 is overexpressed in a variety of human tumors, providing a target for anti-cancer molecular therapies. Here, we employed a 2′-O-methoxyethyl (MOE) splice switching oligonucleotide, SSO111, to induce skipping of exon 15 in HER2 pre-mRNA, leading to significant downregulation of full-length HER2 mRNA, and simultaneous upregulation of Δ15HER2 mRNA. SSO111 treatment of SK-BR-3 cells, which highly overexpress HER2, led to inhibition of cell proliferation and induction of apoptosis. The novel Δ15HER2 mRNA encodes a soluble, secreted form of the receptor. Treating SK-BR-3 cells with exogenous Δ15HER2 protein reduced membrane-bound HER2 and decreased HER3 transphosphorylation. Δ15HER2 protein thus has similar activity to an autoinhibitory, natural splice variant of HER2, Herstatin, and to the breast cancer drug Herceptin. Both SSO111 and Δ15HER2 may be potential candidates for the development of novel HER2-targeted cancer therapeutics. © 2008 Wiley-Liss, Inc.