Ulcerative Colitis (UC) denotes a phenotype of chronic inflammatory bowel disease (IBD), where inflammation is localized to the colonic mucosa, and extends from the rectum proximally in varying extents. The disorder is thought to result from an inappropriate activation of the mucosal immune system by antigens derived from both the host epithelium and the enteric flora, in genetically susceptible individuals. UC is strikingly heterogeneous with respect to age of onset, anatomical extent and disease course, with some experiencing chronically active severe disease, while others have intermittent periods of clinical remission and disease exacerbation. Patients' therapeutic responses vary. The reasons underlying such variability are not well understood. Although it has been widely hypothesized that several genes may influence the development of UC, and modify its phenotypic expression and severity, to date there are few confirmed examples of such relationships.