|Selenomethionine is a naturally occuring amino acid in some plant materials such as cereal grains, soybeans and enriched yeast but it cannot be synthesized from animals or humans. It can be produced from post-structural modifications. *In vivo*, selenomethionine plays an essential role in acting as an antioxidant, where it depletes reactive oxygen species (ROS) and aids in the formation and recycling of glutathione, another important antioxidant. In comparison to selenite, which is the inorganic form of selenium, the organic form of selenomethionine is more readily absorbed in the human body. Selenomethionin is used in biochemical laboratories where its incorporation into proteins that need to be visualized enhances the performance of X-ray crystallography.