A mutagen is any agent (physical, chemical or biological) capable of altering the structure of DNA within our cells. Whilst some mutation is a natural process resulting from errors in the copying and repair of DNA, mutagens are able to greatly increase the mutation rate within a population. Some mutagens naturally exist in the world around us, for example UV radiation, cosmic rays and some radioactive isotopes. Others are specific chemicals that have been synthesised for use in manufacturing or other industries. Mutagens may also arise during the metabolism of certain foods. In general mutagens act either directly on the DNA themselves or produce chemicals that then act directly on the DNA to cause substitutions, deletions, insertions, or rearrangements of the important genetic code. If DNA alterations occur in germline cells (egg or sperm), the altered DNA may become the basis for an inherited disease.