PPARD and colonic neoplasm: Although questions are co-dependent on the potential role of endogenous ligands of PPARβ/δ, which are known to exist and cause chronic, PPARβ/δ activity in many mammalian cells, the focus of the remainder of this review is on the relative expression of PPARβ/δ in cancer and what has been learned in the past 20 years, with an emphasis on colon cancer.