Hey Boy! You Look Mighty Good in Them Genes!
Regulation of Transcription

The problem is, you can’t just go around constantly transcribing all your genes, converting your entire genetic repertoire into protein. That would be a mess. You don’t need to make all your proteins all the time, any more than you need to have all your software running on your computer all the time. What’s more, (ahem), your genes need to fit. The genes you need to express in your liver aren’t the same as the genes you need to express in your neurons, even though both cell types possess all the same genes. So you need to be able to switch genes on and off for normal growth, development and function.