The p53 gene is mutated in ~50% of human cancers and these mutations often contribute to poor responses to cancer therapy. The tumour suppressor p53 is a master regulator of several cellular processes, which together, mediate the suppression of tumorigenesis. The p53 protein functions as a homo-tetrameric transcription factor, which directly transcriptionally regulates ~500 genes that suppress tumorigenesis, including the expression of p21, which is critical for the induction of cell cycle arrest/cell senescence, and the Puma gene, which is a critical initiator of apoptotic cell death. To investigate p53-mediated activation of these pathways, we have created two reporter mouse models, where GFP is knocked into the p21 locus behind an IRES, or in which the Puma coding region was replaced with the sequence for tdTomato. These reporter mice can be used to provide information on p53-mediated transcriptional activation of the different tumour suppressor processes by using FACS analysis and microscopic imaging. We aim to validate both mouse reporter lines and investigate p53-induced cell fate in vitro and in vivo. Validation has been carried out using FACS analysis, with activity of both reporters observed in mouse dermal fibroblasts (MDFs) and thymocytes following treatment with the MDM2 inhibitor nutlin-3a or the DNA damage inducing chemotherapeutic drug etoposide. Further validation is being carried out in diverse cell types following treatment with various agents, using the IncuCyte and Lattice Lightsheet microscopes. The reporters are also being investigated using intravital imaging of live mice, allowing reporter activity to be detected in diverse cell types in specific organs (e.g., skull and spleen). Following validation, we will inter-cross the p21 and Puma reporter mice to get information on why certain cells undergo cell cycle arrest/senescence after p53 activation, while others undergo apoptotic cell death. Do individual cells only activate transcription of the critical initiator of one of these processes, or both? This will provide valuable new insight into the mechanisms by which p53 prevents tumour development.