As discussed in Chapter 6, the expression of specific genes in particular cell types or tissues is regulated by DNA sequence motifs present within promoter or enhancer elements. These elements control the alteration in chromatin structure of genes that occurs in a particular lineage, or the subsequent induction of gene transcription. It was assumed for many years that such sequences would act by binding regulatory proteins that were only synthesized in a particular tissue or were present in an active form only in that tissue. In turn, the binding of these proteins would result in the observed effect on gene expression. Indeed, as described in Chapter 6 (Section 6.3) cell extracts can be used in DNA mobility shift or DNase I footprinting assays to show that they contain protein(s) able to bind to a specific sequence.<\/p>\n\n\n\n