To further demonstrate the biological relevance of stroma-derived Nox4 in breast cancer tumorigenesis and metastasis, we determined the tumorigenicity of E0771, a murine mammary tumor cell line, when orthotopically implanted into either the wild type C57BL/6 mice (Nox4+/+) or the KO mice with a constitutive deletion of Nox4 (Nox4−/−). The gene discussed is NOX4; the disease is breast cancer.