Steroid hormones regulate growth and development in certain types of cells and are involved in a wide variety of physiological responses in most tissues. These responses depend on the association of a hormone with a receptor protein and the subsequent activation of different genes at precise stages during embryonic development and cell differentiation. The primary role of the steroid receptor complex is to bind to specific loci in the genome and regulate rates of gene transcription, but in addition steroids can also modulate the expression of some genes posttranscriptionally. The nuclear binding sites are specific DNA sequences termed steroid response elements that, upon steroid binding, function as transcriptional enhancers. After this interaction there are specific changes in the structure of chromatin which correlate with transcription of the gene and may mediate the action of the steroid. There are a variety of transcriptional enhancers associated with certain genes which interact with activating proteins and these could account for the expression of different networks of genes in individual cells that respond to different physiological stimuli.