2NTI:
reference: Structures of monomeric, dimeric and trimeric PCNA: PCNA-ring assembly and opening., Hlinkova V, Xing G, Bauer J, Shin YJ, Dionne I, Rajashankar KR, Bell SD, Ling H, Acta Crystallogr D Biol Crystallogr. 2008 Sep;64(Pt 9):941-9. Epub 2008, Aug 13. PMID: 18703842

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