Some forms of mutations are hereditary (germline mutations), or passed on from a parent to its offspring. Such mutated coding regions are present in all cells within the organism. Other forms of mutations are acquired (somatic mutations) during an organisms lifetime, and may not be constant cell-to-cell. These changes can be caused by mutagens, carcinogens, or other environmental agents (ex. UV). Acquired mutations can also be a result of copy-errors during DNA replication and are not passed down to offspring. Changes in the coding region can also be de novo (new); such changes are thought to occur shortly after fertilization, resulting in a mutation present in the offsprings DNA while being absent in both the sperm and egg cells.