Cysteine (cysteinyl residue) modifications in proteins result in diversity in protein functions. The reaction specificity of a protein with a modified cysteine residue is determined by the overall conditions of the protein, including the spatial position of the cysteine residue, electrostatic interactions between cysteine residue and other charged residues, spatial interactions between the cysteine residue and a chemical compound, electrophilicity of the chemical compound, and the pH of the solution. In cysteine-dependant enzymes, each specific type of cysteine modification characterizes the catalytic mechanism of the enzyme. Recently, the catalytic mechanisms of peroxiredoxins and cysteine proteases, which contain a cysteine residue(s) in their catalytic sites, have been elucidated. In the catalytic process of peroxiredoxins, a sulfenyl intermediate is formed by oxidation of the catalytic cysteine residue. On the other hand, in cysteine proteases, the catalytic cysteine residue reacts with the carboxyl carbon of a peptide substrate to form an intermediate complex via Salkylation. In this review, we introduce the most current information on the applications of cysteine thiol chemistry for in vitro glycoprotein synthesis. Recently, a glycoprotein (monocyte chemotactic protein-3), containing an intact human complex- type sialyloligosaccharide has been chemically synthesized. The procedure used for this could have applications in the development of new protein-based drugs, including antineoplastic drugs and antibiotics. It can also potentially be applied for improving the half-life and reducing the toxicity of these drugs, and for preventing the development of multidrug resistance.