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The initial lesions formed in DNA by ionizing radiation include base damage, single strand breaks (SSB), double strand breaks (DSB), DNA cross links, and deletions. Deletions occur through energy deposition and perhaps more importantly through recombination repair of DSB's. Several mechanisms for the formation of DSB's and deletions related to energy deposition can be considered. Track simulation codes have indicated the importance of clusters of ionizations in small volumes similar to the size of a nucleosome. These clusters have been related to several types of damage to DNA, including DSB and deletions resulting from multiple DSB's formed by single electron tracks.The deletion size expected from clusters can be estimated at 2-100 bp as constrained by the wrapping of DNA about histones in the nucleosome and expected cluster regions of <5 nm. A second mechanism for deletion results from the higher order structure of DNA. Single ion tracks passing through cells will intersect several segments of DNA and deletions of kbp size as related to chromatin structure are expected and have recently been measured6. In heavy ion irradiation, the high densities of ionizations leads to the overlap of electron tracks suggesting an alternative mechanism for the formation of DSB's or deletions. For electron or photon irradiation, the contribution of electron overlap in causing DNA damage has been estimated to be small at doses below 106 Gy. The radial distribution of dose from secondary electrons exceeds 106 Gy near an ions path and the lateral region of such energy deposition may extend to distances >100 nm for large ion charge suggesting an electron overlap contribution for formation of DSB's or deletions.