The role of cyclooxygenase-2 (COX-2) in colorectal tumorigenesis in mice was studied by Oshima et al. to determine the effects of COX-2 gene knockouts and a new COX-2 inhibitor. In the study, heterozygous Apc^j716 knockout mice, a mouse model of human familial adenomatous polyposis (FAP), were either crossed to COX-2 gene knockout mice, or fed chow containing the COX-2-selective inhibitor. Apc^j716 litter mates were used as positive controls, which developed 652 - 198 (SD) polyps at 10 weeks. Introduction of a COX-2 gene mutation, or feeding with the COX-2-selective inhibitor to the Apc^j716 knockout mice, reduced the number and size of intestinal polyps dramatically. The results provide direct genetic evidence that COX-2 plays a key role in tumorigenesis, and indicate that COX-2-selective inhibitors can be a new class of therapeutic agents for colorectal polyposis and cancer.