Making a Histone Mark The covalent marks on histones (the principal components of chromatin) play a critical role in the regulation of gene expression. Somehow these marks are preserved when a cell in a tissue divides so that the daughter cells maintain the gene expression program and tissue identity of the parent cell. Jacob et al. (p. 1249 ) show that the Arabidopsis histone methylase ATXR5 is specific for the replication-dependent histone variant H3.1 and maintains the repressive histone H3 lysine-27 methyl mark on the H3.1 variant during genome replication, thus, preserving cell-type–specific regions of heterochromatin and gene repression through cell division and beyond.