Cyclophilin 40 also known as, Cyclophilin D, CYP-40, PPID is a member of the peptidyl-prolyl cis-trans isomerase (PPIase) family. Cyclophilin 40 plays a crucial role in protein folding and isomerization, specifically catalyzing the cis-trans isomerization of proline imidic peptide bonds in oligopeptides, which is essential for proper protein function and stability. Cyclophilin 40 is predominantly located in the cytoplasm, where Cyclophilin 40 interacts with various client proteins, including steroid hormone receptors, thereby influencing their activity and stability. Cyclophilin 40′s ability to modulate the function of these receptors highlights Cyclophilin 40′s importance in cellular signaling pathways and stress responses, making Cyclophilin 40 a significant target for research in areas such as cancer and autoimmune diseases.