Is it ensure, insure, or assure?
word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," related to genos "birth" (see genus). Originally in late 18th century chemistry and probably reflecting misunderstanding of -genes, as though it meant "that which produces."
-gen or -gene
One that is produced: phosgene.