Crystal structure of glutaminyl tRNA synthetase (GlnRS, green) in complex with its substrate tRNA^Gln (yellow). Right panel: Color-coded residues depict favorable (blue) and unfavorable (red) effects on the free energy of glutamine binding from mutation at this position. Left panel: Effects of mutation on the ability of GlnRS to catalyze amino acid attachment to the tRNA. In this case all effects are unfavorable. Credit: Annia Rodriguez/ John Perona / UCSB