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The complex locus ade10 of Schizosaccharomyces pombe was subjected to genetical fine structure analysis and characterized for enzymatic activities. Out of twenty alleles isolated, fifteen were found to complement and to be localized in two adjacent regions. Complementing alleles mapping in region I lack AICAR-formyltransferase activity while complementing alleles mapping in region II lack IMP-cyclohydrolase activity. In both cases, the remaining activity was generally normal. Three of these alleles were identified as missense and one as nonsense. Interallelic complementation was found to occur inside each region. The five other alleles are completely pleiotropic, suggesting that the two regions belong to a single transcription unit. These alleles are distributed over both regions. Two of them appear to be frameshift. The genetic map shows the occurrence of map expansion.