1XT5:
reference: Ancient evolutionary origin of diversified variable regions demonstrated by crystal structures of an immune-type receptor in amphioxus., Hernandez Prada JA, Haire RN, Allaire M, Jakoncic J, Stojanoff V, Cannon JP, Litman GW, Ostrov DA, Nat Immunol. 2006 Aug;7(8):875-82. Epub 2006 Jun 25. PMID: 16799561

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