2LR2:
reference: Lanthanide binding and IgG affinity construct: Potential applications in solution NMR, MRI, and luminescence microscopy., Barb AW, Ho TG, Flanagan-Steet H, Prestegard JH, Protein Sci. 2012 Jul 31. doi: 10.1002/pro.2133. PMID: 22851279

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