My textbook tells me that it’s specific transcription factors that allow for a different set of genes to be expressed in different cells (differential gene expression). My book gives the example of liver and lens cells—liver cells contain the specific transcription factors (STFs) to activate the enhancer for the albumin gene, while lens cells contain the STFs to activate the enhancer for the crystallin gene. Liver cells do not have the STFs to activate the crystallin gene, and lens cells cannot activate the album gene.
However, my book also tells me that there is a basal level of transcription that arises from the binding of general transcription factors. Enhancers only increase the rate of transcription on top of that basal rate. Does this mean that in differentiated cells, all genes are indeed transcribed? I.e. a lens cell does transcribe the gene for albumin and the liver cell does transcribe the gene for crystallin?
(The wording of my book dissuades me: "Liver cell: The albumin gene is expressed, and the crystallin gene is not. Lens cell: The crystallin gene is expressed, and the albumin gene is not.”)