Mutant C22 contained three substitutions that considerably increased the size of the apical loop thus shortening the base paired stem. Three nucleotide substitutions present in mutant C23 had the opposite affect in shrinking the apical loop (Figures 4) . Both C22 and C23 had altered growth dynamics, plaque morphology and cpe (Table 1 and Figure 5 ). The nucleotide substitutions of both led to amino acid substitutions Q 32 !R and V 31 !S, respectively. To exclude their influence on virus growth, counterpart 'control' mutants R 32 and S 31 were analyzed, with the same amino acid substitutions but without alteration of the SL6 conformation (Figures 3 and 4) . Both of these control mutants exhibited wild-type plaque morphology and cpe characteristics (Table 1) .