We have molecularly characterized the SNG1 gene that confers hyper-resistance to the mutagen N-methyl-N'nitro-N-nitrosoguanidine (MNNG) in Saccharomyces cerevisiae when overexpressed on a multi-copy plasmid. This hyper-resistance to MNNG is not due to depletion of glutathione pools since multi-copy SNG1 containing yeast transformants contain at least wild type levels of glutathione; DNA repair seems unaffected in these transformants as the multi-copy SNG1-mediated MNNG hyper-resistance is also seen in DNA repair mutants belonging to each of the three epistasis groups of yeast repair mutants. It could be shown that SNG1 is not under control of the YAP1 encoded transcription activator that controls expression of at least two genes involved in MNNG metabolism in yeast. sng1 null mutants are viable but exhibit only slight sensitivity to MNNG, indicating that SNG1 does not encode a protein involved in a major detoxification step of this mutagen. Sequencing of the HYR-mediating passenger DNA revealed that SNG1 encodes a 547 a polypeptide containing seven transmembrane-spanning regions that may be membrane-bound. Comparison of the DNA sequence with established gene databanks revealed that SNG1 is a novel yeast gene.