Both liver cells and lens cells have the genes for making the proteins albumin and crystallin, but only liver cells make albumin (a blood protein) and only lens cells make crystallin (the main protein of the lens of the eye). The specific transcription factors made in a cell determine which genes are expressed. In this example, the genes for albumin and crystallin are shown at the top of the photo, each with an enhancer made up of three different control elements. Although the enhancers for the two genes both have a gray control element, each enhancer has a unique combination of elements. All the activator proteins required for high-level expression of the albumin gene are present in liver cells only (left of the photo), whereas the activators needed for expression of the crystallin gene are present in lens cells only (right of the photo). For simplicity, we consider only the role of specific transcription factors that are activators here, although repressors may also influence transcription in certain cell types.
Urry, Lisa A.. Campbell Biology (p. 374). Pearson Education. Kindle Edition.