Being arboviruses, MBFVs and TBFVs are adapted for transmission between distantly related vertebrate hosts and invertebrate vectors. The requirement to adapt to different molecular environments might result in the evolution of enhancer elements essential for virus replication in one host while being redundant in another. This could explain the contradiction between strict conservation of the different flavivirus enhancers and their apparent redundancy in laboratory systems, which are largely based on mammalian cells (17, 18, 23, 27, 43, 44) . Mutations in SL6 described here have demonstrated its enhancer properties in mammalian cells and it will be interesting to evaluate SL6 enhancer activity in ticks, the major host for maintenance of the TBFV group in the environment (59) (60) (61) .