Several sequentially located genes (an operon) can be co-repressed. Evidence is weak for operons in higher organisms, but all the genes in one of the two entire X chromosomes in female mammalian cells are silent. A mammalian unit of epigenetic co-regulation has been suggested (which could be named a “chromeron”), as based upon chromosomal co-localization of sets of genes involved in apoptosis and in responsiveness to retinoids.29 Repression has been comprehensively reviewed. 30