<p> Coactivators are molecules that have recently been found to be important components of the transactivation activity of steroid hormone receptors. The coactivators were originally defined as factors that increase the total amount of induced gene product with saturating, or pharmacological, concentrations of hormone. For the p160 Src-1 coactivator this action is thought to involve domains with intrinsic transactivation activity, called activation domains. Their intrinsic transactivation activity may be related to their histone acetyltransferase (HAT) activity. The ability of coactivators to augment the total amount of induced gene transcripts requires coactivator binding to the agonist-bound receptors and may result from more efficient binding/recruitment of transcription complex core factors. The COOH-terminal active segment of SRC-1 is devoid of activation domains but does contain a HAT activity domain and a receptor interaction domain. This segment also displays a dominant negative repressor activity, which appears to be due simply to its ability to bind tightly to the receptor ligand binding domain, thereby blocking the binding of other coactivators [<cite idref="PUB00010636"/>]. </p>