The tumor suppressor p53 acts as a stress activated transcription factor that directs the expression of genes involved in growth control. A number of signaling pathways are controlled by p53, including DNA damage response, cell cycle control and induction of apoptosis. The p53 protein is tetrameric and contains several binding sites of cellular proteins that regulate p53 activity. P53 is an instable and conformative flexible protein, therefore activation and inactivation are likely modulated by induction of conformational changes of the p53-protein. Cyclophilins are proteins which exhibit PPIase activity, which is essential to protein folding in vivo, and play a role in regulation of transcription and differentiation. The tumor suppressor protein p53 physically and functionally interacts with Cyp18. This interaction requires the active site of Cyp18 and reduces the sequence specific DNA binding of p53. Cyp18 preferably binds to the proline-rich region of p53, since: (i) a synthetic peptide comprising amino acids 68-81 of p53 inhibited this interaction; (ii) the respective p53 sequence fitted into the Cyp18 active site by molecular docking, and (iii) a p53 variant containing a proline residue at position 72 (p53P72), which is widespread in equatorial regions, interacted with Cyp18 more effectively than the corresponding p53R72 variant. Impairment of the Cyp18-p53 interaction by admitting CsA to p53-expressing culture cells induced a G2/M-phase arrest that was more pronounced when p53P72 rather than p53R72 was expressed in an otherwise isogenic cellular background. Moreover, the expression of p53-dependent genes, like Mdm2, was influenced by the administration of CsA. The functional interplay between these two proteins may be of medical importance since the cyclophilin 18-inhibitor Cyclosporin A (CsA) is widely prescribed to patients after organ transplantations. Unfortunately, these patients have an up to 20% likelihood to develop UV-induced skin carcinomas.