Repressive histone methylation: a case study in deterministic versus stochastic gene regulation.

Transcriptionally repressive histone lysine methylation is used by eukaryotes to tightly control cell fate. Here we explore the importance of this form of regulation in the control of clustered genes in the genome. Two distinctly regulated gene families with important roles in vertebrates are discussed, namely the Hox genes and olfactory receptor genes. Major recent advances in these two fields are compared and contrasted, with an emphasis on the roles of the two different forms of histone trimethylation. We discuss how this repression may impact both the transcriptional output of these loci and the way higher-order chromatin organization is related to their unique control.