mutagen

from WordNet (r) 3.0 (2006)

mutagen
    n 1: any agent (physical or environmental) that can induce a
         genetic mutation or can increase the rate of mutation

from The Collaborative International Dictionary of English v.0.48

mutagen \mu"ta*gen\ (m[=u]"t[.a]*j[e^]n), n.
   A chemical compound or other external influence (such as
   ionizing radiation) which causes mutations[3].
   [PJC]