The gene ade6 is located on chromosome III of the fission yeast Schizosaccharomyces pombe. It codes for the enzyme phosphoribosylaminoimidazole carboxylase involved in purine biosynthesis. A DNA fragment of 3043 nucleotides has been sequenced. It complements ade6 mutations when present on plasmids. An uninterrupted open reading frame of 552 amino acid residues was identified. A method for the cloning of chromosomal mutations by repair of gapped replication vectors in vivo has been developed. Twelve ade6 mutant alleles have been isolated. The sequence alterations of four mutant alleles have been determined. Among them are the ade6-M26 recombination hot spot mutation and the nearby ade6-M375 control mutation. Both are G to T base substitutions, converting adjacent glycine codons to TGA termination codons. They are suppressed by defined tRNA nonsense suppressors of the UGA type. The ade6-M26 mutation leads to a tenfold increase of the occurrence of conversion tetrads in comparison with other ade6 mutations. Possible explanations for the M26-induced increase of recombination frequency are discussed in relation to specific features of the nucleotide sequence identified in the region of the M26 mutation.