Figure 1.

Two models for the role of the different histone modifications. (a) Histone modifications are linked, such that all active genes are marked by an identical combination of 'active' histone modifications, and all inactive genes by another common set of 'repressive' histone modifications. (b) Histone modifications are not linked but form a complex combinatorial code, such that various combinations of modifications mark (groups of) active genes, and various other combinations mark inactive genes. Note that some organisms (such as budding yeast) may lack 'repressive' histone modifications.