**5.2 Nonessential VZV ORFs**

As previously mentioned, 26 of the 70 unique VZV ORFs were deemed nonessential for VZV replication in MeWo cells (Zhang et al., 2010). Of these, 8 ORFs appeared to significantly affect viral growth. In viral growth assays, the peak signals from their corresponding plaques were at least 5-fold less than the peak signals from the WT parental strain. Furthermore, atypical morphology of virally infected cells, such as reduced plaque sizes and altered syncytia formation, were also frequently observed. Studies have shown that some of these ORFs affecting optimal growth encode the small and large subunit of ribonucleotide reductase (Heineman & Cohen, 1994) and specific phosphoproteins that are post-translationally modified by protein kinases (Reddy et al., 1998).

The plaques corresponding to the remaining 18 nonessential VZV ORF deletions exhibited wild-type growth in cultured MeWo cells (Zhang et al., 2010). *In vitro* growth curve analysis for viral replication showed that these ORF deletion mutants have the same growth kinetics as their wild-type parental strain, VZVLuc. Nonessential genes, in general, are significantly enriched for other and unknown functional categories (Fig. 9B).
