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The influence of duplicated sequences on chromosomal stability is poorly understood. To characterize chromosomal rearrangements involving duplicated sequences, we compared the organization of tandem repeats of the DUP240 gene family in 15 Saccharomyces cerevisiae strains of various origins. The DUP240 gene family consists of 10 members of unknown function in the reference strain $288C. Five DUP240 paralogs on chromosome I and two on chromosome VII are arranged as tandem repeats that are highly polymorphic in copy number and sequence. We characterized DNA sequences that are likely involved in homologous or nonhomologous recombination events and are responsible for intra-and interchromosomal rearrangements that cause the creation and disappearance of DUP240 paralogs. The tandemly repeated DUP240 genes seem to be privileged sites of gene birth and death.

Chromosomal rearrangements, such as deletions, duplications, or Ty transposition, are rare events. We devised a method to select for such events as [Ura.sup.+] revertants of a particular ura2 mutant. Among 133 [Ura.sup.+] revertants, 14 were identified as the result of a deletion in URA2. Of seven classes of deletions, six had very short regions of identity at their junctions (from 7 to 13 bp long). This strongly suggests a nonhomologous recombination mechanism for the formation of these deletions. The total [Ura.sup.+] reversion rate was increased 4.2-fold in a rad52 [Delta] strain compared to the wild type, and the deletion rate was significantly increased. All the deletions selected in the rad52 [Delta] context had microhomologies at their junctions. We propose two mechanisms to explain the occurrence of these deletions and discuss the role of microhomology stretches in the formation of fusion proteins.