All three engineered mutants demonstrated delayed growth in PS cells, most visible during the first 24 hours (Fig. 4) . Nevertheless, they all subsequently reached titres similar to pGGVs HA-positive virus by day 3 (Table 1) . Although, mutant IC-D67G exhibited better growth characteristics than IC-E122G and IC-D277A it was reproducibly slightly lower than the control pGGVs virus.