In terms of the order of appearance of the individual substitutions, the presence of GU wobble base pairs in two tombusviruses (Cucumber necrosis virus (CNV) and TBSV-statice isolate (TBSV-S); refer to Figure 8A ) suggests that the A-to-G substitution at the distal AS1 occurred first, followed by a compensating U-to-C at RS1 ( Figure 10B ).