4OPY:
reference: Negative Epistasis and Evolvability in TEM-1 beta-Lactamase--The Thin Line between an Enzyme's Conformational Freedom and Disorder., Dellus-Gur E, Elias M, Caselli E, Prati F, Salverda ML, de Visser JA, Fraser JS, Tawfik DS, J Mol Biol. 2015 Jul 17;427(14):2396-409. doi: 10.1016/j.jmb.2015.05.011. Epub, 2015 May 22. PMID: 26004540

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