Cyclophilin A, also known as Peptidylprolyl Isomerase A, PPIA, CYPA, and CYPH, was originally characterized for its ability to catalyze the transition between cis and trans proline residues critical for proper folding of proteins. Ubiquitously expressed, Cyclophilin A has a predicted molecular weight of 17 kDa and is proinflammatory cytokine that is secreted in response to inflammatory stimuli. Human Cyclophilin A shares 95% amino acid identity with both the mouse and rat orthologs. Cyclophilin A is the main target of the immunosuppressive drug Cyclosporin A. The immunosuppressive activity of cyclosporins has been correlated with their ability to form complexes with cyclophilins that inhibit Calcineurin Phosphatase activity and prevent incorporation of Cyclophilin A into viral particles. Cyclophilin A is known to be incorporated into many viruses, including HIV1 and Hepatitis C, where it may be involved in functions such as viral assembly, replication, and infectivity. In addition, Cyclophilin A has been shown to promote atherosclerosis via binding to Extracellular matrix metalloproteinase (MMP) inducer (EMMPRIN) on CD34+ progenitor cells and stimulating differentiation to foam cells, an essential step during atherosclerotic plaque formation in blood vessels. Cyclophilin A-induced disruption of blood-brain barrier function is thought to contribute to the increased risk of developing Alzheimer’s disease in individuals that possess one or two Apolipoprotein E4 alleles.