- The alpha 2 type IX collagen gene tryptophan polymorphism is not associated with rheumatoid arthritis in the Japanese population. Takata, Y., Matsui, Y., Hamada, D., Goto, T., Kubo, T., Egawa, H., Nakano, S., Shinomiya, F., Inoue, H., Itakura, M., Yasui, N. Clin. Rheumatol. (2006)