The 20,349-bp sequence of the human cytomegalovirus (HCMV) HindIII F fragment has revealed eight open reading frames with homology to herpes simplex virus (HSV) and/or Epstein-Barr virus (EBV). With respect to EBV, these homologous genes can be divided into two blocks: one block contains three genes, including the DNA polymerase and glycoprotein B, and the other block contains five genes of unknown function. Although the relative organisation of genes within each block is identical in HCMV and EBV, the relative position of each block within the two genomes differs: in HCMV the two blocks are present directly adjacent to each other, whereas in EBV they are found 92 kb apart. This suggests that a genetic rearrangement has occurred in this region. Transcription analysis of the glycoprotein B gene is presented and the evolutionary relationship between the genomes of HCMV, EBV, and HSV is discussed.