Cyclophilins (CyPs) constitute a large class of highly conserved, ubiquitous PPIases (EC 22.214.171.124), which together with FK560 binding proteins (FKBP) and parvulins belong to a superfamily of immunophilins. These three classes of proteins are easily distinguishable by their selective interactions with immunosuppressive drugs. Cyclophilins are targets for cyclosporin A (CsA), parvulins bind juglone (5-hydroxy-1.4-naphthoquinone) and FKBP are inhibited either by FK560 drug or by rapamycin. Despite the lack of structural similarity all these proteins have been shown to act as peptidylprolyl cis-trans isomerases. In all cases binding of the drug inhibits their PPIase activity. Distinct isoforms of cyclophilins have been localized in the cytoplasm, nucleus, mitochondria, chloroplasts and endoplasmic reticulum.