2′-Deoxyribonucleotide 5′-triphosphates were obtained from New England Biolabs. 3′-Amino-2′,3′-dideoxyribonucleotide 5′-triphosphates were obtained from Trilink Biotechnologies. DNA oligonucleotides were purchased from IDT. Oligonucleotides incorporating a 3′-amino residue were synthesized and purified by standard methods, using phosphoramidites obtained from Chemgenes or as recently reported (13 (link)) and using reagents purchased from Glen Research on an Expedite 8909 synthesizer. Premixed reagent solutions for crystallization were purchased from Hampton Research. Recombinant lysozyme and Bugbuster lysis reagent were purchased from EMD. Ni-NTA Superflow affinity resin was purchased from Qiagen. All other buffers and reagents were prepared from high-purity chemicals obtained from Sigma-Aldrich, using chemicals with trace metals analysis where available. Competent cells were obtained from New England Biolabs.