[en] Insult from ionizing radiation is always in the form of individual structured tracks. At cell level possible responses are limited by numbers and variety of tracks and on time intervals between them. Most situations of practical concern in radiation protection are characterized by single tracks well separated in time from any other tracks in the same cell. A wide variety of initial molecular damage is possible, at the level of DNA, due to the traversal of just a single track. Therefore no dose-threshold can be expected for such molecular damage. A crucial question is how the cell may respond to such damage, received in isolation, and how this response may be modified by additional uncorrelated damage elsewhere in the same cell but received a considerable time earlier or later. (author)