**7.1 Screening for VZV neurotropic factors**

We proposed that the VZV genome also encodes factors required for efficient invasion and egress from specific tissues during natural infection, such as neurotropic factors. Using our newly created VZV deletion mutants, we screened each of the 18 dispensable VZV ORFs to determine which are implicated with VZV neuronal infection. First, the mutant DNAs were confirmed to be replication competent in a primary permissive cell line, the human retinal epithelial ARPE19 cells (Schmidt-Chanasit et al., 2008). Then, the deletion mutants were screened in human neuroblastoma, SH-SY5Y, following a similar transfection approach as described earlier for MeWo cells, to establish their essentiality for replication in human neurons.

Of the mutants tested, only the ORF7 deletion mutant was unable to form viral plaques (data not yet published). To confirm this, we also infected differentiated neuroblastoma and human embryonic stem cell-derived neurons with wild type and 7D cell-free particles. As expected, the WT infection exhibited robust proliferation, while the 7D infection yielded no visible plaques. Our finding here establishes ORF7 as the only known VZV factor required for viral spread in human neurons.
