1. Departments of Physics and Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA

Correspondence to: S. Leibler Correspondence and requests for materials should be addressed to S.L. (e-mail: Email: leibler@princeton.edu).

Cells use complex networks of interacting molecular components to transfer and process information. These "computational devices of living cells"¹ are responsible for many important cellular processes, including cell-cycle regulation and signal transduction. Here we address the issue of the sensitivity of the networks to variations in their biochemical parameters. We propose a mechanism for robust adaptation in simple signal transduction networks. We show that this mechanism applies in particular to bacterial chemotaxis^{2, 3, 4, 5, 6, 7}. This is demonstrated within a quantitative model which explains, in a unified way, many aspects of chemotaxis, including proper responses to chemical gradients^{8, 9, 10, 11, 12}. The adaptation property^{10, 13, 14, 15, 16} is a consequence of the network's connectivity and does not require the 'fine-tuning' of parameters. We argue that the key properties of biochemical networks should be robust in order to ensure their proper functioning.