Posttranslational modifications, which influence protein function, include highly complex glycan modifications. The structure of the modified glycans is diverse, subject to various controls, and imparts a wide range of functions to proteins. This is because there are many glycan-recognizing molecules, collectively called lectins, in the body, and the function of glycoproteins changes depending on whether they are recognized by these lectins. Lectins were originally found in large numbers in plant seeds, but it has since been revealed that they are also abundant in our bodies and perform various functions. To alter the function of proteins in the body, there are direct methods such as modifying the glycan structure of glycoproteins and approaches to modify the glycan recognition of lectins, which form a pair with them. The latter is the main purpose of lectin engineering (Fig. 12.1).