Formyl-L-methionine is a protein amino acid present in the organelles of bacteria and related eukaryotes. It is a derivative of the amino acid methionine, in which a formate is added to the amino group of the methionine. It plays a crucial role in protein biosynthesis in bacteria, Mitochondria and chloroplasts. But it is not used for protein biosynthesis in Eukaryotic Cytosol, in which only the nuclear genes are translated. It's also not used in ARCHAEA. In humans, it is recognized by the immune system as a foreign substance and stimulates the body to fight potential infections.