Cellular memory and the histone code.

The histone tails on the nucleosome surface are subject to enzyme-catalyzed modifications that may, singly or in combination, form a code specifying patterns of gene expression. Recent papers provide insights into how a combinatorial code might be set and read. They show how modification of one residue can influence that of another, even when they are located on different histones, and how modifications at specific genomic locations might be perpetuated on newly assembled chromatin.