||P.69 left column:"DNA suffers oxidative damage from free radicals generated by ionising radiation as well as from reactive oxygen species such as those inevitably arising in respiration. This damage takes the form of single-strand breaks (SSB), much less abundant double-strand breaks (DSB), alkali-labile sites, and various species of oxidised purines and pyrimidines (van Sonntag, 1987). DNA breaks in living cells are fairly easily measured, and the time course of their repair has been studied. SSB rejoining is generally rapid, with a half-time (t1/2) for repair of a few minutes in normal cells (see for review, primary source). DSB repair is slower, although estimates of the rate vary, depending on the method used to measure them in general, t1/2 is greater than 1 h (primary source). Repair of oxidised bases is more difficult to study, since standard chemical methods are not sufficiently sensitive to measure individual oxidation products formed after biologically tolerable doses of damage."