5UEF:
reference: Insight into the mechanism of nonenzymatic RNA primer extension from the structure of an RNA-GpppG complex., Zhang W, Tam CP, Walton T, Fahrenbach AC, Birrane G, Szostak JW, Proc Natl Acad Sci U S A. 2017 Jul 18;114(29):7659-7664. doi:, 10.1073/pnas.1704006114. Epub 2017 Jul 3. PMID: 28673998

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