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PROTEINS called histones play such an important role in regulating genes that
we should think of them as a 鈥渉istone code鈥�, complementary to the genetic code,
says biochemist David Allis of the University of Virginia.

鈥淔or some time now, we have known there is more to our genetic blueprint than
DNA itself,鈥� says Allis. Individual genes can be turned off by adding methyl
groups to DNA, a process called imprinting. What鈥檚 more, imprinted DNA can be
passed from one generation to another, effectively passing down information that
isn鈥檛 directly encoded in our genome.

DNA molecules wrap themselves around histones, and it has recently emerged
that a variety of chemical changes to these proteins, such as the addition of
acetyl groups, can regulate gene expression. Now an international team including
Allis has found that adding methyl groups to specific parts of a histone called
H3 has a dramatic effect. It doesn鈥檛 just affect single genes, but can turn
large stretches of our genome on or off, says Allis.

In an accompanying paper, Allis and Thomas Jenuwein of the Vienna Biocenter
in Austria go beyond these findings, arguing that the 鈥渉istone code鈥�
considerably extends the information encoded in the genome, and plays a vital
role in determining the fate of cells.

鈥淲e鈥檝e now got our whole genome sequenced鈥攁nd that鈥檚 going to lead to a
lot of important studies,鈥� says Allis. But he points out that studying the
genome alone will never reveal what leads to gene expression or lack of
expression.

Unravelling histone switches will be important for future stem cell work,
says Allis. 鈥淭he goal of being able to make tissues we need isn鈥檛 going to be
much more than fantasy until we understand the consequences of the histone code
over time.鈥�