A change occurring in the genetic material (DNA) in the CHROMOSOMES of a cell. It is caused by a fault in the replication of a cell's genetic material when it divides to form two daughter cells. Mutations may occur in somatic cells, and may result in a local growth of the new type of cells. These may be destroyed by the body's defence mechanism or they may develop into a tumour. If mutation occurs in a germ cell or gamete – the organism's sex cells – the outcome may be a changed inherited characteristic in succeeding generations. Mutations occur rarely, but a small steady number are caused by background radiation in the environment. Since individual genes (or genes in combination) represent the body's code for cells to manufacture proteins, any mutation may result in the absence or excess of a particular protein or a variation in its nature or quality. Mutations can be classified as follows:

• Missense: a change in one DNA base pair so that one amino acid is substituted for another in the protein made by a gene

• Nonsense: rather than substituting an amino acid, the changed DNA sequence signals the cell to stop manufacturing a protein

• Insertion: a piece of DNA is added so changes the number of DNA bases in the gene

• Deletion: a piece of DNA is removed so reducing the number of bases

• Duplication: a piece of DNA is copied once or many times and alters the function of the resulting protein. (See also MUTAGEN and GENETIC DISORDERS.)