Defining an epigenetic code.

The nucleosome surface is decorated with an array of enzyme-catalysed modifications on histone tails. These modifications have well-defined roles in a variety of ongoing chromatin functions, often by acting as receptors for non-histone proteins, but their longer-term effects are less clear. Here, an attempt is made to define how histone modifications operate as part of a predictive and heritable epigenetic code that specifies patterns of gene expression through differentiation and development.