So Serene is polar and reactive in forming disulfide bridges. So if we compare these two, this is almost the same as this. The only difference is we replace the oxygen in this case with a sulfur in this case. But cysteine is actually sometimes known as a special side chain or a special amino acid because of its importance in forming those disulfide bridges. So, as we said earlier, Asparagin and glutamine contain polar carboxyamide groups. Now let's focus on two special cases that contain special side chain groups.