Genotoxic damage is defined as changes in DNA caused by chemical substances or physical effects (e.g. UV or X-rays). These can result in a gene mutation, e.g. by insertion or deletion of one or more bases. DNA repair enzymes can often repair the effects, so not every genotoxic event results in a change in the base sequence. If this is unsuccessful, a mutation may occur, possibly resulting in cancer (carcinogenesis) and malformations (teratogenesis).