To test the biological significance of SL6 in the TBFV group we engineered 21 mutant viruses with point mutations that altered the linear sequence of the unpaired apical loop or destabilized the base-paired stem. Substitutions within the conserved hexanucleotide loop down-regulated virus growth kinetics whereas changes in the terminal loop outside the hexanucleotide sequence did not alter the observed phenotype. The most significant changes of virus phenotype resulted from substitutions that distorted the stem of SL6; mutations that influenced the length or stability of the stem resulted in the recovery of viruses that formed small and/or turbid plaques. Increasing or decreasing the size of the apical loop had a minor biological effect on virus replication although this could also be interpreted as an effect of the altered stem length. However, the changes in replication kinetics from all modifications of SL6 were moderate and manifested themselves predominantly during the early stage of the virus replication cycle (Table 1) .