Not only steroid, but also thyroid hormones and vitamins D and A act trough very similar mechanisms causing the same general effects, i.e. RNK and protein synthesis induction. In some cases hormone-receptor complex, as a transcriptional factor (TF), acts by decreasing gene activity. Gene expression control can be realized on several different levels, the most important one being on the level of transcription initiation. The fact that the same TFs can activate different genes indicates the importance and participation of gene controlling elements in its activation. Stable hormone-receptor complex acts as an activator that binds on different gene controlling elements (hormone responsive elements - HRE). Peptide hormones act via membrane receptors in several ways: either by changing the permeability of cell membrane (the hormone binds to the membrane receptor and create hormone-receptor complex which results in configuration and conformational changes, thus changing membrane permeability for different elements and substances) or through intracellular enzyme activation (second messengers) by hormone binding on the membrane receptor. Second messengers are cAMP, cGMP, Ca ions and calmoduline, and also products of phospholipid membrane lyses. The messenger usually causes further activation of enzyme cascade. Hormones that act via membrane receptors most often act on the activity of cell enzymes which are responsible for synthesis and lyses of different products. Biological effects of hormones depend on their affinity and capability for receptor binding, as well as on the concentration of free hormones in plasma. Knowledge about gene expression regulation is very important for different branches and fields of clinical medicine e.g. oncology, virusology, endocrinology, immunology, and for all others as well. Specific activity of certain genes in specific tissues is very important for cell proliferation and differentiation.