Abstract: The invention described herein provides a recombinant replication-defective vims derived from Herpesvirales order, wherein the virus is characterized by a complete deletion of a gene encoding ICP27, or a functional equivalent gene thereof. The invention also provides production cell lines for such recombinant replication-defective vims, wherein the cell lines have a coding sequence for ICP27 or a functional equivalent thereof, and wherein the coding sequence has no or minimal sequence overlap with the virus characterized by the complete deletion of the gene encoding ICP27. Method of using such recombinant replication-defective vims and production cell lines are also provided.