Abstract
The genome sequencing of complex organisms has reaped a wealth of information about genes. The next step is to find out what the proteins encoded by these genes do and how they interact with each other. In a Perspective, Stuart Kim, discusses the application of yeast two-hybrid technology to working out the complex interactions between the many proteins of the worm C. elegans ( Walhout et al.). This new work is a first step in the creation of a global protein interaction map for the nematode.