Inflammatory bowel diseases are complex diseases due to the combination of genetic and environmental factors. Many genes have a limited effect and none is necessary nor sufficient. It has been observed that many of these genes confer susceptibility to specific clinical forms of the disease. So, HLADRB1*0103 confer susceptibility to the pancolitis form of ulcerative colitis and also to Crohn's disease but only with colonic localization. IKBL gene confer susceptibility only to ulcerative colitis, also the pancolitis form and refractary to treatment. NOD2/CARD15 gene confer susceptibility only Crohn's disease with ileal localization with frequent stenosis.