We have carried out ab initio electronic structure calculations on a portion of DNA, the results of which provide support for a mechanism that produces single-strand breaks (SSBs) with low-energy electrons. This mechanism involves attaching a low-energy electron (ca. 1 eV) to a pi* orbital of a DNA base to form a shape-resonance state. This pi* anion then undergoes a sugar-phosphate C-O bond rupture over a small barrier to produce SSBs. In addition to supporting the efficacy of such a mechanism, our results suggest that solvation plays a crucial role in the rate of SSB formation when such very short-lived shape resonances are involved. In particular, they suggest that either the pi* anion must be rendered electronically stable by solvation or its detachment lifetime must be several orders of magnitude longer in the solvated species than in the nonsolvated species.