Another approach to determine the role of the 2b protein in the generation and/or selection of the recombinant RNAs, was to use a different 2b gene in the heterologous interspecific hybrid virus. To this end, we replaced the whole 2b gene of (the subgroup II strain) Q-CMV with that from a subgroup IA strain of CMV, WAII-CMV, generating the hybrid virus C1C2 W2B T3 (see C2 W2B in Figure 1 ). The WAII-CMV 2b gene and the Q-CMV 2b gene are only 53.4% identical at nucleotide level and the WAII-2b protein is 10 amino acids longer than the Q-2b protein.