The tumor suppressor protein p53 is a sequence-specific transcription factor that has crucial roles in apoptosis, cell cycle arrest, cellular senescence, and DNA repair. Following exposure to a variety of stresses, p53 becomes post-translationally modified, with concomitant increases in activity and stability. To better understand the role of acetylation of Lys<SUB>317</SUB> in mouse p53, the effect of ionizing radiation on the thymocytes of p53<SUP>K317R</SUP> knock-in mice was studied at the global level. Using cleavable ICAT quantitative mass spectrometry, the effect of IR on protein levels in either the wild type or p53<SUP>K317R</SUP> thymocytes was determined. We found 102 proteins to be significantly affected by IR in the wild type thymocytes, including several whose expression has been shown to be directly regulated by p53. When the effects of IR in the wild type and p53<SUP>K317R</SUP> samples were compared, 46 proteins were found to be differently affected (p<0.05). The p53<SUP>K317R</SUP> mutation has widespread effects on specific protein levels following IR, including the levels of proteins involved in apoptosis, transcription, and translation. Pathway analysis of the differently regulated proteins suggests an increase in p53 activity in the p53<SUP>K317R</SUP> thymocytes, as well as a decrease in TNF<IMG SRC="/math/alpha.gif" ALIGN="BASELINE" ALT="alpha "> signaling. These results suggest that acetylation of Lys<SUB>317</SUB> modulates the functions of p53 and influences the cross-talk between the DNA damage response and other signaling pathways.