Ulcerative colitis is a chronic, debilitating disease characterized by damage to the mucous membrane (mucosa) lining the lower gastrointestinal tract. Endoscopic and histological evidence of healing of the damaged mucosa, referred to as mucosal healing, is increasingly accepted as an important endpoint in clinical practice and in clinical trials due to its association with improved clinical outcomes. In fact, in a draft guidance to industry, the Food and Drug Administration encouraged incorporating histologic evaluation in ulcerative colitis clinical trials.