Selenocysteine is an amino acid that makes up our body’s proteins and is important for good health as it helps the body absorb proteins. Selenocysteine is the 21st amino acid in the genetic code, in which selenocysteine is usually the termination codon with unique biosynthetic pathways and properties. Selenocysteine is usually found in a few of the body’s enzymes, such as glutathione peroxidase. Selenocysteine contains selenol groups, which are more easily oxidized and therefore have antioxidant properties in protein molecules. Selenocysteine itself is an indispensable amino acid in the human body and is an important part of proteins, which play an important role in the daily health of the human body. In terms of research, selenocysteine is cytotoxic and can be used as a potential anti-cancer drug.