Altman and colleagues (this issue) call attention to the inability of current standardized enzyme nomenclature to distinguish between enzymatic activities that reside in nonhomologous macromolecules. This issue is highlighted by the fact that the pre-tRNA 5′-maturation activities of bacteria and plant chloroplasts present the first instance (of which I am aware) of two naturally occurring enzymes that cannot be evolutionarily related, but which catalyze an identical reaction. (In the classic example of convergent evolution between the trypsin family and subtilisin, the enzymes do not have an identical substrate specificity.) Altman and colleagues propose that a single trivial name be used only for members of a family of homologous macromolecules; in other words, that different trivial names be given to enzymes that catalyze the same precursor–product conversion but do so with different catalytic mechanisms, or which are not members of a single family of homologous macromolecules.