The purpose of this study is to study samples of rectal mucosa (the moist lining of the rectum) using a new light scattering technology, called partial wave spectroscopy (PWS)in patients who will undergo a colonoscopy or a flexible sigmoidoscopy and have a history of ulcerative colitis (UC). PWS has been recently developed to allow scientists to learn about any changes to the cells that may indicate a risk of developing colon cancer. We believe that the PWS test will more accurately detect the risk of colon cancer.