We previously demonstrated that RON and HGFL play important roles in breast development and tumorigenesis, with alterations in HGFL-RON signaling affecting terminal end bud formation in the developing mammary gland, which contains mammary stem cells, and promoting breast cancer progression and metastasis in transgenic models of breast cancer [6, 17, 18, 27]. The gene discussed is MST1; the disease is breast cancer.