The yeast two-hybrid (Y2H) system can pump out vast quantities of data on a genome of any size. Although simple – it measures the ability of two proteins to find each other and "interact" in the milieu of a living yeast cell – the process revealed a dazzling volume of information about the networks in which specific proteins operate. Y2H assigns greater importance to those proteins that interact with many partners, and less to those with fewer partners. This raised the prospect of using Y2H to map the interactome. Researchers showed the approach worked by mapping a phage, and they honed the technique in the organism in which it is assayed, Saccharomyces cerevisiae. Soon after, Y2H was ready for the challenge of a metazoan.