FKBP4, also known as FK506-binding protein 4 or FKBP52, is a member of the FK506-binding protein (FKBP) family, a group of peptidyl-prolyl cis-trans isomerases (PPIases) that function as molecular chaperones. FKBP4 is primarily found in the cytoplasm and nucleus of cells, where it helps regulate protein folding, trafficking, and signal transduction. A key role of FKBP4 is acting as a co-chaperone for steroid hormone receptors, such as the glucocorticoid receptor (GR) and androgen receptor (AR). FKBP4 interacts with these receptors in the cytoplasm, assisting their proper folding and assembly into functional complexes with heat shock proteins (HSPs).