Updated publication reference for PubMed record(s): 34281173. The aim of this investigation was to study the consequences of interfering with soluble epoxide hydrolase (sEH) expression on tumor growth and metastasis in genetically modified animals that spontaneously generate tumors without the exogenous application of high concentrations of epoxide mediators or inhibitors. Therefore, breast cancer development was studied in mice expressing the polyoma middle T oncogene (PyMT) under the control of the mouse mammary tumor virus promoter, to induce spontaneous mammary tumors. To facilitate the study of endogenous sEH activity in tumor growth, PyMT mice were then crossed with sEH-/- mice to generate sEH-deficient mice that spontaneously generate breast tumors (so called PyMTsEH mice). For these analyses, primary tumors were removed from 20 week old mice.