Currently the DISC-HSV is propagated in a complementing cell line derived from a Vero (African Green monkey kidney) cell line approved by the World Health Organisation (WHO) for use in vaccine manufacture (WHO, 1989) . The Vero cells were modified to contain the HSV-2 gH gene under the control of the HSV-1 glycoprotein D (gD) gene promoter (Boursnell et al., 1997) . The complementing cell line was designated CR2. Because the gD promoter requires additional HSV proteins for its induction, gH is regulated so that it is only produced in the cell following an infection with virus. Virus produced from CR2 cells can infect normal cells, but can only perform one cycle of replication. The progeny virus from this replication cycle lack the gH protein and are therefore non-infectious ( Figure 1 ).