By complementation of a mutant resistant to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) we have identified the gcs1 gene, encoding a putative gamma-glutamylcysteine synthetase. The gene is possibly interrupted by two introns and has 49% identical and 80% similar amino acids compared with the homologous protein from rat. In comparison with the Saccharomyces cerevisiae homologue it possesses 41% identical and 74% similar amino acids.