It is crucial for organisms to be able to control which genes are expressed in which cells and when. Naturally occurring chemical modifications of DNA-binding histone proteins are believed to play an important role in this process; however, it had been unclear whether they play a causal role in instructing gene expression. Researchers have experimentally shown that certain histone sites act as critical control sites that help prevent the spurious activation of parts of the genome, including sequences derived from ancient remnants of viruses.