Many teratogens exert specific effects on the fetus by epigenetic mechanisms.[92][93] While epigenetic effects may preserve the effect of a teratogen such as diethylstilbestrol throughout the life of an affected child, the possibility of birth defects resulting from exposure of fathers or in second and succeeding generations of offspring has generally been rejected on theoretical grounds and for lack of evidence.[94] However, a range of male-mediated abnormalities have been demonstrated, and more are likely to exist.[95] FDA label information for Vidaza, a formulation of 5-azacitidine (an unmethylatable analog of cytidine that causes hypomethylation when incorporated into DNA) states that "men should be advised not to father a child" while using the drug, citing evidence in treated male mice of reduced fertility, increased embryo loss, and abnormal embryo development.[96] In rats, endocrine differences were observed in offspring of males exposed to morphine.[97] In mice, second generation effects of diethylstilbesterol have been described occurring by epigenetic mechanisms.[98]