Mutation C15, which shortened the apical loop (Figure 4) , did not affect virus growth but changed the plaque morphology and delayed the development of cpe (Table 1 and Figure 5 ). The C15 mutation altered the amino acid M 33 !L, which imitates the KFDV/AHFV group (Figure 3) , potentially contributing to the observed biological effect. Mutation C21 that reduced the apical loop size (Figure 4 ) interfering with exposure of the hexanucleotide, also had a moderate affect on virus growth although the accompanying effect of amino acid substitution Q 32 !H (Figure 3 ) cannot be excluded (Table 1 and Figure 5 ).