Put it all together and you get a dizzying array of transcriptional activators, promotors, repressors, enhancers  and other players; a repertoire of strategies for the cell to exploit so as to exquisitely regulate its genetic expression. So a liver cell can turn on liver genes by expressing liver-specific transcription factors which bind only to promotors and enhancer elements of those genes. Neurons express a library of neuronal transcription factors so they only turn on the genes that neurons need to express.

Of course, all those transcription factors are, themselves, coded for by genes that also have to be regulated and have their own transcriptional control elements. And many of these transcription factors are regulated by enzymes, which are also coded for by genes with their own transcriptional control elements. And so on, and on, and on. Deal with that!