The protein p53, often called the ‘guardian of the genome,’ is a transcription factor that is activated in response to cellular stress (low oxygen levels, heat shock, DNA damage, etc.) and acts to prevent further proliferation of the stressed cell by promoting cell cycle arrest or apoptosis. Its role as a tumor suppressor is evident by the observation that approximately 50% of human tumors have mutated or non-functional p53. Mdm2, a key negative regulator of p53, which is over-expressed in many human tumors, functions by binding to and targeting p53 for proteasomal degradation. Nutlin-3 is a potent inhibitor of p53-Mdm2 interaction. (+)-Nutlin-3 is arbitrarily referred to as enantiomer b because it appears as the second peak from chiral purification of racemic nutlin-3 and its absolute stereocenter assignment is not known. It is 150-fold less potent as an inhibitor of p53-Mdm2 interaction than (-)-nutlin-3, demonstrating an IC50 value of 13.6 µM. This inactive enantiomer can serve as a useful control for non-Mdm2 related cellular activities.