Nucleosomes at active promoters: unforgettable loss

A variety of chromatin features have been implicated in the regulation of gene expression, including nucleosome occupancy at promoters, histone modifications and variants, and DNA methylation. In this issue of Cancer Cell, Lin and colleagues use a powerful single-molecule approach to explore the relationship between nucleosome occupancy and gene expression in cancer cells. They show that nucleosome occupancy is mostly all-or-none at the multiple start sites of the MLH1 CpG island. After demethylation by drug treatment, nucleosomes are permanently lost as transcription becomes reactivated. Thus, epigenetic maintenance of gene expression may require that promoters are maintained free of nucleosomes.