Most of the control of tissue-specific gene expression occurs at the level of transcription; this is achieved with tissuespecific transcription factors. E.g., all the genes that are to be expressed in erythroid cells (such as globins, spectrin, and the erythropoietin receptor) have the site -AGATA- in their promoters. This promoter is only active when bound by a transcription factor, called GATA-1, which is present only in erythroid cells.
