Examples of proteins and protein complexes cross-linked by disulfide bonds. Protein backbones are represented as ribbon traces, and the Cβ and sulfur (yellow) atoms of cysteine side chains are shown as spheres. In protein complexes, different polypeptides are shown in distinct colors. PDB codes are as follows: conotoxin, 2LXG; theta-defensin, 5INZ; antifreeze protein, 1EZG; Izumo, 5JK9; ribonuclease A, 3MZR; laminin fragment, 4AQS; insulin, 2BN3; C4b-binding protein oligomerization domain, 4b0f; Kremen1-LRP6-Dickkopf complex, 5FWS.