In the heteromolecular system, only one type of heteroduplex supporting non-homologous crossovers could possibly form: between wtRNA1 and Mag1-RNA3. In the mixed system, recombining molecules were capable of forming three types of heteroduplexes: (i) intermolecular, between 5 0 RAS1as from MatNH-RNA3 and RAS1s from wtRNA1, (ii) intermolecular, between 5 0 RAS1as and 3 0 RAS1s located in two MatHN-RNA3 molecules and (iii) intramolecular, between 5 0 RAS1as and 3 0 RAS1s located in the same MatNH-RNA3 molecule (Figure 4) . To determine the molecules that participated in non-homologous recombination, the RT-PCR amplified 3 0 -portions of RNA3 were checked for DXho. It was present in 90% of recombinants. This result clearly showed that non-homologous crossovers almost always involve one (intramolecular recombination) or two (intermolecular recombination) MatNH-RNA3 molecules.