The identification of off-target binding of drugs is a key to repositioning drugs to new therapeutic categories. Here we show the universal interactions of the p53 transactivation domain (p53TAD) with various antiapoptotic Bcl-2 family proteins via a mouse double minute 2 (MDM2) binding motif, which play an important role in transcription-independent apoptotic pathways of p53. Interestingly, our structural studies reveal that the anti-apoptotic Bcl-2 family proteins and MDM2 share a similar mode of interaction with the p53TAD. On the basis of this close molecular mimicry, our NMR results demonstrate that the potent MDM2 antagonists Nutlin-3 and PMI bind to the anti-apoptotic Bcl-2 family proteins in a manner analogous to that with the p53TAD.