Yeast two-hybrid systems provide a sensitive method for detecting relatively weak and transient protein interactions. Such interactions may not be biochemically detectable, but may be critical for proper functioning of complex biological systems. pGBKT7-53 and pGADT7-T encode fusions between the GAL4 DNA-BD and AD and murine p53 and SV40 large T-antigen, respectively. p53 and large T-antigen interact in a yeast two-hybrid assay.