Since one-gene metabolic engineering will necessarily reach its limits when coping with today's increasingly complex challenges, the recent development of artificial eukaryotic operons enables effective multigene metabolic engineering of mammalian cells. This greatly expands possibilities to reprogram interconnected cellular networks in desired ways to improve key characteristics of mammalian cells. Besides use in next-generation multigene metabolic engineering, multicistronic expression units are expected to have great impact on very specific applications including (i) straightforward combinatorial evaluation of gene functions and metabolic networks, (ii) one-step transfection, selection and maintenance of difficult-toexpress (multi-subunit) proteins, (iii) selection of high producer cell lines, and (iv) genetic immunisation and gene therapy in combination using sense, antisense or ribozyme technology. Production of high titre disabled infectious single cycle (DISC) HSV from a microcarrier culture Disabled Infectious Single Cycle (DISC) HSV-2 has been cultured in the complimentary cell line CR2 to provide high titre bulk material suitable for the purification of the virus as a live viral vaccine. CR2 cells are cultured on the microcarrier Cytodex-1 at 5 g l-1 in small scale (1 l) and larger scale (15 l) reactors. The cells are infected at an MOI of 0.01 pfu cell-1 and the culture harvested 60–72 h later. The infected cells are removed from the microcarriers by the addition of a hypotonic saline and the virus released by low-pressure disruption techniques. Virus titres achieved are compared to the standard roller bottle process. The resulting material is the starting point for the purification of the DISC-HSV virus. The incidence of genital herpes is high and increasing world-wide (Corey, 1993 ). An important aim in combating the disease is achieving an effective vaccine which can act against both the primary and recurrent disease caused by herpes simplex virus. The company strategy has therefore been to adapt live viruses by genetic manipulation to introduce an acceptable margin of safety through the development of DISC (Disabled Infectious Single Cycle) virus vaccines. These are viruses which lack an essential gene, and are therefore unable to undergo multi-cycle replication in a vaccinated host. They can, however be prepared in a way that allows them to go through a single cycle of replication in cells of the vaccinee, leading to an effective immune response. It has been shown that herpes simplex viruses lacking the essential glycoprotein H (gH) gene can be used as effective vaccines (Farrell et al., 1994; McLean et al., 1994) . The virus is therefore genetically inactivated so that it is unable to spread within the host. * Part of this work was presented as a poster at the ESACT meeting in Tours, France. September 1997.