Nature Genet. doi:10.1038/ng.154 (2008)

The sections of its DNA that a cell expresses — and thus the cell's characteristics — depend in part on chemical modifications to the histone proteins around which DNA is wound. A set of 17 such modifications is associated with a quarter of all promoters — gene-regulatory sites — in human immune cells, find Keji Zhao of the US National Institutes of Health in Bethesda, Maryland, and his colleagues.

They looked at the different combinations of chemical alterations that affect the expression of about 12,500 genes in CD4+ T cells. One type of modification — acetylation — does not directly determine whether a gene is 'read', as had been suspected, but seems to prime the gene for activation.