The human herpesvirus 6 (HHV-6) variant A genome has conserved sequences which are signals for initiating lytic replication (origin, 'ori-lyt') and DNA packaging into the virion (pac2/1). Here these are functionally characterised and used to construct a gene-expression amplifiable-vector, an 'amplicon', with applications for gene delivery to lymphoid-myeloid cells or their progenitor stem cells. A minimal efficient ori-lyt for replication was identified which was enhanced in the presence of the imperfect direct repeated DNA domain (IDR). In A variant strains these are arranged as three adjacent repeats with the most divergence in IDR3. Addition of the pac2/1 sequences also enhanced detection of ori-lyt replication and conferred DNA packaging properties, thus, the amplicon could be packaged with 'helper' virus. An HHV-6 specific factor, which inhibits amplicon replication was identified by trans replication assays. This is the U94-Rep 'latency' gene product, which can modulate efficiency of such amplifiable vectors, based on the lytic origin. It could also affect maintenance of viral genomes or vectors during latency.