The first indication of the cooperation between ErbB2 and TGFβ in mammary cancer comes from an in vivo study where MMTV-Neu (expressing rat ErbB2 under MMTV promoter) mice are crossed with MMTV-TGFβ mice expressing constitutively active, mutated TGFβ [109] or TGFβRI [110]. The gene discussed is ERBB2; the disease is breast cancer.