Abstract
Herpes simplex virus 1 deleted for VP22 is proposed to require secondary mutation of VHS for viability. Here we show that a replication-competent Δ22 virus constructed by homologous recombination maintains a Wt VHS gene and has no other gross mutations. By contrast, Δ22 viruses recovered from a bacterial artificial chromosome contain multiple codon changes within a conserved region of VHS. Hence, the mode of virus rescue influences the acquisition of secondary mutations.