It would seem likely that the polymerase is as equally able to use RNA 1 as RNA 2 for the receptor RNA, since the polymerase is essentially the same in all cases, and the presence or absence of the 2a protein C-terminal sequences (where the recombinant polymerases showed some differences in sequence) did not affect the nature of the recombinant RNAs. However, since only one type of recombinant progeny was observed in each case, this demonstrates that there must have been selection for a particular progeny RNA, which depended on the source of the 2b gene and its encoded protein.