Transcription is regulated by activators and repressors, proteins that bind to DNA. In eukaryotes, these regulatory proteins often need additional proteins, called co-regulators, to function properly. These co-regulators bind to activator or repressor complexes; however, they do not recognize cis-regulatory sequences, as they cannot bind directly to DNA. A transcriptional regulator binds to a cis-regulatory sequence before the co-regulator associates; regulators and co-regulators often cannot form stable complexes unless the regulator is bound to DNA. In certain cases, RNA molecules also serve as scaffolds to hold all the proteins in a complex together. Some of these co-regulators are enzymes that can regulate gene expression, such as histone acetyltransferases and deacetylases. Acetyltransferases are co-activators that transfer an acetyl group to the histone leading to the loosening up of the DNA and promoting transcription. Deacetylases are co-repressors that remove acetyl groups resulting in the tight packing of DNA around the histones and preventing transcription. For example, the co-regulator SMRT associates with the thyroid hormone receptor at the positive hormone response element and acts as a co-repressor to inhibit basal transcription. When a hormone binds to the receptor, the SMRT dissociates, and a co-activator binds to activate transcription. However, when bound to the same receptor on a negative hormone response element, SMRT activates transcription in the absence of the hormone. Thereby acting as a co-activator.