Nuclear receptors are transcription factors that regulate gene expression in a ligand-dependent manner. They play key roles in several basic biological processes such as growth, differentiation and maintenance of homeostasis. According to the current general model of nuclear receptor action, in the absence of ligand, co-repressor and other members of the repressor complex are bound to the receptor, keeping the target gene silent. However, in the presence of an agonist ligand, co-regulator exchange takes place, which means that the repressor complex is released and co-activator with members of the activator complex takes its place. This regulator exchange results in activation of target gene expression. This process is viewed as the molecular switch model, which represents two distinct states of a rather static system. Due to modern biophysics and the evolution of microscope technology, new applications became available in the field of molecular biology, which made it possible to investigate transcriptional regulation at increasingly higher time resolution. As the result of these experiments the earlier static model is being replaced by a rather dynamic one.