Yar-strains of TBEV i.e. Yar71, Yar 114, Yar 46-2, and Yar 48 were isolated in the European part of Russia (Yaroslavl' region) between 1999-2001 and details for their isolation are listed in Table 1 . All 4 Yar viruses had equivalent infectivities when compared with the Vasilchenko (Vs) strain of TBEV (see Methods) used as the positive control virus (Table 1 ) and produced comparable concentrations of E protein when analysed by Western blot (data not shown). However, they were completely negative in HA tests over a range of pH 5.75-7, regardless of whether they were prepared in newborn mice or in PS cells. The control Vs virus and pGGVs virus, recovered from the infectious clone (see Methods), produced high positive HA titres (1:640-1:1280) at pH 6.2 and this was therefore the pH of choice for all subsequent tests (Table 1) . Thus, the 4 Yar isolates can be defined as HA-or AD-deficient in common with the other 40 strains that have been isolated in Europe and were also identified as HA-and AD-deficient [21] .