The control of development operates at many different levels. In recent months this has become easy to forget, with the nucleus the focus of attention and the DNA-binding properties of the homoeo box taking much of the limelight. Now the sequencing of two other 'controlling' gene, one from insects and the other from nematodes, has revealed homology with mammalian proteins of very different function. The lin-12
gene of Caaenorhabditis elegans, which controls patterns of cell lineage, and the Notch gene of Drosophila, which plays a role in the early development of the nervous system, both encode proteins homologous with epidermal growth factor (EGF) and the structurally related family of mammalian proteins. Other members of this family are all membrane-bound or secreted proteins, suggesting that Notch and lin-12
act at or beyond the cell membrane, perhaps to mediate cell communication.