Proteins are traditionally identified on the basis of their individual actions as catalysts, signalling molecules, or building blocks in cells and microorganisms. But our post-genomic view is expanding the protein's role into an element in a network of protein–protein interactions as well, in which it has a contextual or cellular function within functional modules^{1, 2}. Here we provide quantitative support for this idea by demonstrating that the phenotypic consequence of a single gene deletion in the yeast Saccharomyces cerevisiae is affected to a large extent by the topological position of its protein product in the complex hierarchical web of molecular interactions.

1. Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA
2. Department of Pathology, Northwestern University Medical School, Chicago, Illinois 60611, USA

Correspondence to: A.-L. Barabási¹ e-mail: Email: alb@nd.edu

Correspondence to: Z. N. Oltvai² emails: Email: alb@nd.edu and Email: zno008@nwu.edu