In our studies on genetic RNA recombination we have used the well-characterized in vivo system developed in BMV (30, 33) . BMV is a model (+)RNA virus of plants (38) . Its genome is composed of three segments called RNA1, RNA2 and RNA3. RNA1 and RNA2 encode BMV replicase proteins 1a and 2a, respectively. RNA3 encodes movement (3a) and coat proteins (CP) (38) . All three BMV RNAs possess an almost identical 3 0 -untranslated region (3 0 -UTR). The first BMV-based recombination system was created by Nagy and Bujarski (33) . They constructed a recombinationally active BMV mutant whose genome is composed of wtRNA1, wtRNA2 and modified RNA3 (PN0-RNA3 called the recombination vector, for details see Figure 1 ). Only 3 0 -UTR was modified in PN0-RNA3, while its 5 0 -UTR, intergenic and coding regions were unchanged. Despite the introduced changes, the recombination vector is stable and replicates when used together with wtRNA1 and wtRNA2 to infect plants. It starts to recombine if a recombinationally active sequence (RAS) is introduced just between the CP coding sequence and the modified 3 0 region (into the RAS-cloning site). Non-homologous recombination was observed when a 140-60 nt sequence complementary to RNA1 between positions 2856 and 2992 was inserted into PN0-RNA3 (a sequence from the 3 0 -portion of RNA1 was introduced in antisense orientation) (30, 33) . Interestingly, the same RNA1 fragment inserted in sense Figure 1 . The BMV-based recombination system. White, black and gray boxes represent coding, noncoding and recombinationally active sequences, respectively. The location of the primers (A and B) used for specific RT-PCR amplification of the 3 0 -portion of BMV RNA3 (parental or recombinant) is indicated by arrows. (A) BMV genome. The BMV genome consists of three RNA segments: RNA1, RNA2 and RNA3. All three BMV RNAs share an almost identical 3 0 -noncoding region with a tRNA-like structure at the very end. (B) Recombination vector. The PN0-RNA3 vector is a wtRNA3 derivative with a modified 3 0 -noncoding end [(for details see ref. (33) ]. The latter includes (i) the RAS cloning site, (ii) a 197 nt sequence derived from the 3 0 -noncoding region of cowpea chlorotic mottle virus RNA3 (marked as CCMV), (iii) the sequence of wtRNA3 between nt 7 and 200 (counting from the 3 0 end-marked as the region B) and (iv) the last 236 nt from the 3 0 end of BMV wtRNA1 (marked as A). (C) Non-homologous recombination. Non-homologous recombination was observed if an 140 nt sequence, called RAS1as (shown in A), complementary to wtRNA1 between positions 2856 and 2992 was inserted into the PN0-RNA3 vector. The presence of the RAS1as sequence in RNA3 derivative (called Mag1-RNA3) allows local RNA1-RNA3 hybridization that mediates frequent non-homologous crossovers. It is thought that polymerase starts nascent strand synthesis on the 3 0 end of RNA1 and then switches to RNA3 within a local double-stranded region. (D) Non-homologous recombinant. Non-homologous recombination repairs Mag1-RNA3 by replacing its modified 3 0 end with the 3 0 -noncoding fragment coming from RNA1. orientation did not support homologous crossovers (33) . Non-homologous recombination repaired the RNA3 vector by replacing its highly modified 3 0 end with 3 0 -UTR derived from RNA1. The resultant recombinants replicated and accumulated better than the parental RNA3 molecule, and so the latter was out competed from the infected cells. The above system is extremely efficient, since it employs selection pressure to support the accumulation of RNA3 recombinants. Because RAS is placed in two different segments of the BMV genome, we have proposed to name this system heteromolecular.