There are still many unknowns about how diverse glycans are biosynthesized and how individual processes of glycan synthesis and completed glycans are recognized by other molecules and function in the body. To elucidate the molecular mechanisms of glycan biosynthesis and glycan recognition, three-dimensional structural information of the proteins involved is essential. In recent years, in addition to X-ray crystallography, single-particle analysis by cryo-electron microscope (Cryo-EM) and high-precision structural prediction represented by AlphaFold [1] have been added as methods for obtaining three-dimensional structural information. Cryo-EM is not an ideal method for glycosyltransferases and lectins as it is difficult to measure samples below 100 kDa. AlphaFold accurately predicts the three-dimensional structure of the protein itself, but there are challenges in predicting the interaction sites of compounds, and the structure of glycans protruding from the protein surface is extremely difficult due to the structural degrees of freedom of the glycans themselves [2]. The elucidation of the molecular mechanism of glycan synthesis, which was a challenge in the book published in 2018, has not been fully clarified, but it is steadily being elucidated using X-ray crystallography (see Fig. 20.1 for an example).