The MNNG hyper-resistance of yeast transformants containing multiple copies of the SNQ3/YAP1 yeast gene is not caused by lowered MNNG activation due to depleted pools of glutathione. On the contrary, the SNQ3/YAP1-encoded protein stimulates production of GSH, apparently by promoter activation due to the AP-1 recognition element. Expression of at least one further gene, encoding a protein with a strong detoxifying activity, must also be stimulated to explain the MNNG hyper-resistance phenotype.