Working stocks of HSV-1 SC16 were grown on HaCaT cells and assayed on Vero cells [17] . Mutant viruses lacking functional genes for gH (SCgHZ) [14] , gB (HFEMdUL27-lacZ) [18] or gD (SC16gDdelZ) [18] were grown and assayed on helper cell lines expressing the corresponding glycoprotein: CR1 cells, expressing gH [19] ; D6 cells expressing gB [13] ; and VD60 cells expressing gD [18] . All stocks were grown using an MOI of 0.1.