To ensure their survival, cells must carefully maintain the structure of their most prized possession — DNA. The DNA chain is wrapped around nucleosomes, which consist of histones, and is packaged into a higher order structure, chromatin. Control of this dynamic structure allows transcription to be tightly regulated, and two reviews in this issue describe how this can be done. On page 409, Robert Kingston and Nicole Francis describe how cells mark chromatin so that they can remember which genes should be 'on' or 'off' after division. And Ronen Marmorstein (page 422) discusses how changes in the structure of chromatin can be triggered by modification of the histone tails.

Another way for cells to increase their chances of survival is to respond to external threats, such as damaging environmental agents or pathogens. Environmental agents can lead to breaks in the DNA chain, and a ubiquitous class of enzymes involved in DNA repair and recombination is described in the Review by David Lilley and Malcolm White on page 433. Pathogens, on the other hand, attack the cell as a whole, often using a Trojan Horse line of attack. Some can subvert host defences by integrating cellular genes into their own genomes. src — the subject of the Perspective by G. Steven Martin on page 467 — is a classic example of such a gene. And on page 457, Cesare Montecucco and Rino Rappuoli discuss how another pathogen, Helicobacter pylori, has evolved to overcome a particularly hostile environment — the human stomach.

This month, we welcome the new addition to our team, Katrin Bussell, who will mainly be responsible for signal transduction, adhesion and development.