... following superinfection with all of the otherviruses, including the ICP4-deficient mutant d120,despite the fact that ICP4 is stringently required for expres-sion of VP16 and other HSV E and L genes ... activates transcription of the IE genes. Four of the IE proteins (ICP0, ICP4, ICP22 and ICP27) then serveto drive further progression into the lytic program. Three of these, ICP4, ICP22 and ICP27, ... levels of IE gene expression duringinfection [3-5], and ICP0 activates expression of IE, E andL genes in transient transfection assays [6-10]. Moreover,expression of ICP0 in trans substantially...