1) Drug or radiation effects on DNA that interfere with normal function, including conformational distortions and covalent gene mutations.

2) Drug- or radiation-induced injuries in DNA that introduce deviations from its normal double-helical conformation. These changes include structural distortions which interfere with replication and transcription, as well as point mutations which disrupt base pairs and exert damaging effects on future generations through changes in DNA sequence. If the damage is minor, it can often be repaired (DNA REPAIR). If the damage is extensive, it can induce apoptosis.