All animals, by definition, have common features. So do these indicate that underlying them there is a basic ' dictionary' of genes? There are suggestions that this may be so and that the ' dictionary ' may contain as few as 5,000 genes. Dr Sydney Brenner of the Medical Research Council Molecular Biology Laboratory at Cambridge discusses this hypothesis with John Maddox and also some of the other implications of his efforts to understand the genetic foundation of development. Editor DAVID PATERSON