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Steroid receptors are ligand-dependent intracellular proteins that stimulate transcription of specific genes by binding to specific DNA sequences following activation by the appropriate hormone. Prior to activation, steroid receptors associate with a number of different proteins in both a stable and transient fashion. Steroid receptor complex proteins include heat shock proteins (HSP 70 and HSP 90), immunosuppressant binding proteins called immunophilins (the FK 506 binding proteins, FKBP 52 & FKBP 54 and the cyclosporin binding protein, CyP-40) and at least three other proteins termed p23, p60 and p48. p23 along with HSP 70, HSP 90 and p60, combine with progesterone receptor (PR) as members of a transient intermediate complex. Cloned human p23 encodes a protein of 160 amino acids that is highly conserved between species and shows no homology to previously identified proteins. p23 is a highly acidic phosphoprotein with an aspartic acid-rich C-terminal domain and multiple potential phosphorylation sites. In vitro studies have suggested that p23 binds to HSP90 and is necessary for the binding of HSP 90 and CyP-40 to PR. While neither its exact function nor mechanism of action have been identified, p23 appears to be an important factor in PR function.