Illustration of protein chain topologies that can be formed by different arrangements of SpyTag and SpyCatcher within protein chains. A: SpyTag, B: SpyCatcher, and E: ELP. Reprinted with permission from Journal of the American Chemical Society, Controlling Macromolecular Topology with Genetically Encoded SpyTag-SpyCatcher Chemistry, 135, 2013, Wen-Bin Zhang, Fei Sun, David A. Tirrell, Frances H. Arnold. Copyright 2013, American Chemical Society.