ERBB2 and neoplasm: Analyses of these genetically engineered mice demonstrate that Δ16HER2 constitutively homodimerizes on the tumor cell plasma membrane and is able to transform mammary epithelium in vivo through the oncogenic properties mediated by the downstream Src kinase signaling circuitry, making this splice variant a likely candidate for the transforming form of the HER2 oncoprotein.