Growth curves of wild-type and mutant TBE virus strains determined on primary chicken embryo cells infected at an m.o.i. of 1. The values plotted were derived from duplicate experiments. ▪, Neudoerfl; ▴, R-Nd/3′Δ10795; ▵, R-Nd/3′Δ10826; ○, R-Nd/3′Δ10847; •, R-Nd/3′Δ10870; □, R-Nd/3′Δ10919.