4EFX:
reference: Insulin analog with additional disulfide bond has increased stability and preserved activity., Vinther TN, Norrman M, Ribel U, Huus K, Schlein M, Steensgaard DB, Pedersen TA, Pettersson I, Ludvigsen S, Kjeldsen T, Jensen KJ, Hubalek F, Protein Sci. 2013 Mar;22(3):296-305. doi: 10.1002/pro.2211. Epub 2013 Jan 17. PMID: 23281053

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