This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Low levels of ionizing radiation (IR) causing damage to DNA have always been present in our environment. Living systems have evolved rapid mechanisms to detect and repair damage using special repair enzymes and other proteins. The biological response to low dose IR is difficult to identify and quantify. Many IR-induced genes have been identified, but mRNA expression levels correlate poorly with active protein levels. Cellular response to low levels of IR is therefore better quantified as a change in the protein expression, rather than the direct damage to genomic DNA or the mRNA expression. The high sensitivity of accelerator mass spectrometry (AMS) combined with the specificity of antibodies against targeted putative IR-induced proteins enables detection and quantitation of protein expression at low levels and short times after radiation exposure, for instance in small, well-defined isolates from cell cultures or animal models exposed to IR. This project will provide a new research tool for quantitative protein expression analysis, bridging the gap between global proteomics and interactions of individual DNA damage proteins and their response to radiation, chemical or biological factors.