With molecular biologic and molecular cytogenetic techniques, we study the mutagenic and epigenetic mechanisms that silence transgene expression in a unique set of mammalian cell lines. The G12 and G10 cell lines each contain a bacterial xanthine-guanine phosphoribosyl transferase (gpt) gene target that has been stably inserted into different chromosomal genome locations. These cell lines exhibit differing propensities for mutagenesis and deletions by oxidative carcinogens and clastogens, including x-rays and anticancer therapeutics, e.g., bleomycin and amsacrine, with the G10 cells showing the most sensitivity to these compounds. In contrast to clastogen mutagenesis, some traditionally nonmutagenic carcinogens such as nickel, diethylstilbestrol (DES), and 17b-estradiol have been found to inactivate gpt expression solely or predominantly in the G12 but not G10 cells. Epigenetic silencing of the gpt transgene by DNA condensation and excess methylation has been identified as the mechanism for trangene silencing in the nickel-induced G12 mutants. Preliminary data suggest that this is also true for many DES-induced mutants. Similar studies are being performed for other natural and synthetic estrogens, as well as for putative xenoestrogens such as organic pesticides.