**6. Identification of VZV tissue tropic genes**

Although 26 VZV ORFs are shown to be dispensable for viral replication in cultured MeWo cells, it is possible that some of these viral genes may encode proteins critical for optimal viral infection in skin tissue. To test this hypothesis, all of the nonessential ORF deletion mutants were further tested in human fetal skin organ culture (SOC). SOC is a reliable alternative to the SCID-hu mouse model (Taylor & Moffat, 2005), and is especially convenient for initial genome-wide screening of skin-tropism determinants.

We found that all VZV deletions which demonstrated severe growth defects in cultured MeWo cells also exhibited the same growth defects in SOC samples. Interestingly however, among the 18 VZV ORFs believed to be completely dispensable for viral replication in cultured MeWo cells, four ORFs displayed significant growth defects in SOC (Fig. 8) (Zhang et al., 2010). Because these ORFs are trivial for viral replication in MeWo cells but prove crucial for optimal viral replication in skin tissue, they evoked further investigation as potential skin tropism factors.

Rescue viruses were generated for two of these four ORF deletions to ensure that growth defects in skin culture are in fact due to the functions of the deleted genes. As expected, the growth curve analyses showed that in MeWo cells, rescue viruses grew indistinguishably from wild-type VZV, and in SOC, they were able to fully recover the growth defects of their corresponding deletion mutants (Zhang et al., 2010). Three of these four ORFs (ORF10, ORF14, ORF47) have previously been identified as tissue-tropic factors (Cohen & Seidel, 1994; Heineman & Cohen, 1995; Moffat et al., 1998). Our findings verified these previous studies and additionally identified ORF7 as a novel skin-specific virulence factor. To confirm our original finding here, we also produced a premature stop-codon mutant (ORF7S) by mutating the 5th codon from TGT to the TGA stop codon. Like ORF7D, ORF7S displayed wild-type growth in MeWo cells, but had a growth defect in SOC, indicating that ORF7 may function as a VZV skin-tropic factor.
