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Histones are subject to a complex and dynamic set of covalent modifications that are thought to be involved in the modulation of transcription during development, in X chromosome inactivation in female mammals, and in genome stability and meiotic chromosome dynamics. Histone modifications reported to date include acetylation, phosphorylation, methylation, ADP ribosylation, and ubiquitination (Fig. 1). Multiple residues on each of the four core histones have been identified as potential modification sites and some lysine side chains can be either methylated or acetylated. Given the number of sites and the variety of possible modifications, the combinatorial possibilities are extremely large and it is tempting to believe that histone modification has a regulatory role. This led to the histone code hypothesis, which postulated that these modifications may be interdependent and that they may culminate in specific …