region. A. Genetic organization of the HSV-1 RL region. Numbers refer to base positions in the prototype HSV-1 genome, and arrows denote the LAT, L/ST, ICP34.5, and ICP0 primary transcripts. Reiterated DNA sequences in the RL region are denoted by small boxes containing vertical bars. The location of the DNA sequences to which ICP4 homodimers bind in the LAT and L/ST genes is denoted by pairs of black ovals at the 5' end of each gene. B. The ICP0 genes of wild-type HSV-1 and the ICP0- viruses n212 and 0--GFP. The mutation in n212 introduces a 14 bp linker sequence into codon 212 of the ICP0 open-reading frame, which terminates protein translation . The insertion mutation in 0--GFP introduces an ~770 bp green-fluorescent protein (GFP) coding sequence in-frame with the ICP0 gene. The resulting mRNA is predicted to encode the N-terminal 104 amino acids of ICP0 fused to a 14 amino acid linker and 239 amino acids of C-terminal GFP.