So generally, our polar side chains are more reactive than the hydrophobic side chains. Finally, we have cysteine. Now, cysteine is not only polar, but it's also very special because is responsible for forming these very important bonds known as disulfide bonds or disulfide bridges. And these are covalent bonds that exist within protein structures. And they play an important role in actually determining the three dimensional structure of our protein. So cysteine is structurally similar to Serene, except it has a thyl group.