In Yi Zhang's News and Views article “Molecular biology: No exception to reversibility” (Nature 431, 637–639; 2004), there were errors in Fig. 1b. In the side chain of citrulline, a double bond should have been shown between NH and O, rather than between NH and NH2. Several of the connecting atoms are erroneously shown as H rather than N. And the leaving methylamine should have been represented as +NH3CH3 rather than +NH2CH3