- Amino acid polymorphism at residue 71 in HLA-DR beta chain plays a critical role in susceptibility to ulcerative colitis. de la Concha, E.G., Fernandez-Arquero, M., Martinez, A., Vigil, P., Vidal, F., Lopez-Nava, G., Diaz-Rubio, M., Garcia-Paredes, J. Dig. Dis. Sci. (1999)