ENU, also known as N-ethyl-N-nitrosourea (chemical formula C3H6N3O2), is a highly potent mutagen in the mouse. It has a mutation rate of 1 new mutation (at a particular gene) per 700 gametes. It is also toxic a high doses.
The chemical is an alkylating agent, and acts by transferring the ethyl group of ENU to nucleobases (mainly thymine) in nucleic acids. Its main target is the spermatogonial stem cells , from which mature sperm derive from.