 This novel tRNA-aaRS pair ("orthogonal pair") is independent of other synthetases and tRNAs in Escherichia coli, and further possesses some flexibility in the range of amino acids processed, making it an attractive tool to allow the placement of a possibly wide range of functional chemical groups at arbitrarily specified locations in modified proteins.For example, the system provided one of two fluorophores incorporated site-specifically within calmodulin to allow the real-time examination of changes within the protein by FRET spectroscopy, and site-specific introduction of a photocaged lysine derivative.