Histone modifications dictate specific biological readouts.

The basic unit of chromatin is the nucleosomal core particle, containing 147 bp of DNA that wraps twice around an octamer of core histones. The core histones bear a highly dynamic N-terminal amino acid tail around 20-35 residues in length and rich in basic amino acids. These tails extending from the surface of nucleosome play an important role in folding of nucleosomal arrays into higher order chromatin structure, which plays an important role in eukaryotic gene regulation. The amino terminal tails protruding from the nuclesomes get modified by the addition of small groups such as methyl, acetyl and phosphoryl groups. In this review, we focus on these complex modification patterns and their biological functions. Moreover, these modifications seem to be part of a complex scheme where distinct histone modifications act in a sequential manner or in combination to form a "histone code" read by other proteins to control the structure and/or function of the chromatin fiber. Errors in this histone code may be involved in many human diseases especially cancer, the nature of which could be therapeutically exploited. Increasing evidence suggests that many proteins bear multiple, distinct modifications, and the ability of one modification to antagonize or synergize the deposition of another can have significant biological consequences.