Under non-stress conditions monomeric HSF1 is sequestered in a HSP90-containing heterocomplex. FKBP4 (immunophilin) is one of the components of HSP90-chaperone machinery which was found to associate with trimeric, but not monomeric form of HSF1 (Guo Y et al. 2001). Multichaperone complex of HSP90:FKBP4:PKGES3 has been shown to associate with HSF1 trimer through its regulatory domain, and this is thought to repress HSF1 transcriptional activity (Guo Y et al. 2001).