Another study points out that neurotrophin-3, a neurotrophic factor in the nerve growth factor family, with increased expression in brain metastasis, is able to reduce the expression of nitric oxide synthetase mediated by MAP kinase and PI3 kinase signaling pathways and promote the mesenchymal–epithelial transition of breast cancer cells to enhance its proliferation and metastasis ability (Tzeng et al., 2005; Louie et al., 2013). This evidence concerns the gene NTF3 and breast cancer.