Genes that control development by regulating the expression of structural genes responsible for the formation of body components. Some encode transcription factors, which are proteins that interact with regulatory sites of other genes causing activation or repression of developmental pathways. Many other regulatory genes produce noncoding RNAs, which control gene expression in various ways; some degrade target messenger RNA molecules or block their translation; others block transcription of the structural genes by altering chromatin structure (see chromatin remodelling). Much of development in quite different organisms, such as mammals and insects, is controlled by genes that are structurally very similar, thought to have descended from genes in ancient common ancestors. Prime examples are the homeotic genes, such as the Hox genes of mammals. Genomes also contain numerous regulatory sequences, such as enhancers, promoters, and terminators, which are nontranscribed regions that play crucial roles in gene expression.