ENU, also known as N-ethyl-N-nitrosourea (chemical formula C3H6N3O2), is a highly potent mutagen. For a given gene in mice, ENU can induce 1 new mutation in every 700 gametes. It is also toxic at high doses.
The chemical is an alkylating agent, and acts by transferring the ethyl group of ENU to nucleobases (usually thymine) in nucleic acids. Its main targets are the spermatogonial stem cells, from which mature sperm are derived.