How is gene expression controlled in tissues?

Richard Adams - Two basic ways. It's controlled in a positive way by specifically switching on particular genes as a response to signals that particular cells are experiencing. But it's also controlled by reducing the compliment of genes that are available to be switched on by silencing genes in particular places. Where cells in particular tissues become more specialised, they silence more and more of their DNA, and therefore reduce the possible responses that they can have to the signal.