Cyclophilins (CYPs) are a family of proteins named after their ability to bind to ciclosporin (cyclosporin A), an immunosuppressant which is usually used to suppress rejection after internal organ transplants.

[2][3][4] Major isoforms have been found within single cells, including inside the Endoplasmic reticulum, and some are even secreted.

[5] Cyclophilin D (PPIF, note that literature is confusing, the mitochondrial cyclophilin is encoded by the PPIF gene), which is located in the matrix of mitochondria, is only a modulatory, but may or may not be a structural component of the mitochondrial permeability transition pore.

As a result, the mitochondria fall into a functional disorder, so the opening of the pore plays an important role in cell death.

However, mitochondria obtained from the cysts of Artemia franciscana, do not exhibit the mitochondrial permeability transition pore [8][9] Overexpression of Cyclophilin A has been linked to poor response to inflammatory diseases, the progression or metastasis of cancer, and aging.