**2. Generation of a VZVBAC**

Viral BACs are created when a BAC vector sequence is inserted into the viral genome. Typically, a BAC vector is roughly 10-kb long and contains an origin of replication, genes essential for BAC replication, and genes to control the rate of replication; ideally the copy number should be limited to one or two BACs per bacterial cell (Warden et al., 2010). An antibiotic resistance marker and selection marker, such as a green fluorescent protein, are also added to select for bacterial colonies harboring the BAC vector and isolate these BAC-containing recombinant viruses. BAC vectors in addition must also be flanked by a 500-1000-bp sequence homologous to the target sequence at the site of insertion. Lastly, loxP sites are commonly included at both ends of the BAC sequence to excise the vector after recombinant viruses are generated, as is required for vaccine production (Zhang et al., 2007, see below).
