Any one of the inverted sequences present on the 2-µm DNA from Saccharomyces cerevisiae can promote replication of chimeric plasmids in Schizosaccharomyces pombe. When however a complete 2-µm molecule is present on the transforming plasmids, these are very unstable and systematically rearranged in S. pombe. Two types of transformation are observed in this case. One results from chromosomal integration of the incoming DNA. The second is dependent on a site specific recombination event between two molecules of the incoming DNA and results in a stably replicating dimeric structure. The choice between both pathways seems to depend on the expression of 2-µm function(s) in S. pombe. 