tokens: Three experiments using 20 microM 2 - ( hydroxyamino ) - 1 - methyl - 6 - phenylimidazo [ 4 , 5 - b ] pyridine ( N - OH - PhIP ) were performed to induce mutations in the dihydrofolate reductase ( DHFR ) gene of a hemizygous Chinese hamster ovary ( CHO ) cell line ( UA21 ) .