Figure 6. Non-invasive demonstration of protein–protein interactions using in vivo PET. The radiotracer [9-(4-[¹⁸F]Fluoro-3-hydroxymethylbutyl)guanine] is trapped inside tumour cells that bind p53 and TAg (transforming factor SV40 large T antigen) protein. Gal4 is a DNA binding domain and VP16 an enhancer domain. Binding of p53 and TAg initiates expression of herpes simplex thymidine kinase that phosphorylates the radiotracer and traps it intracellularly. Intracellular trapping is demonstrated via persistent activity over the transformed murine tumour on the right. The EGFP domain can be induced to express green fluorescent protein providing fluorescent confirmation of VP16 TAg binding. Reproduced with permission [53].