Ghosh et al. obtained PPARβ/δ−/−COX-2-TG transgenic mice by crossbreeding and found that PPARβ/δ silencing antagonized cyclooxygenase-2 (COX-2)-induced mammary gland hyperplasia and tumorigenesis in mice and significantly inhibited the expression of breast epithelial cell proliferation-related genes (e.g., Ki-67, Cyclin D1, etc.), revealing that PPARβ/δ plays the role of tumor suppressor in the development of breast cancer [146]. The gene discussed is PPARD; the disease is neoplasm.