In all herpesviruses a block of genes is present which is composed of the genes encoding DNA polymerase, glycoprotein B (gB), ICP18.5 and major DNA-binding protein (MDBP). Here we report the cloning and sequencing of this gene block from rat cytomegalovirus (RCMV). The gene block spans 13.3 kbp and contains the four genes in the order pol, gB, ICP18.5 and MDBP. A similar order of genes has previously been reported for human and murine cytomegaloviruses. The pol, gB, ICP18.5 and MDBP genes contain open reading frames which have the capacity to encode proteins of 1120, 914, 893 and 1281 amino acids, respectively. Comparison of the predicted amino acid sequences of the four RCMV proteins with the corresponding proteins of other herpesviruses revealed a close relationship between RCMV and other cytomegaloviruses, which corroborates the usefulness of the RCMV-rat model for studying cytomegalovirus biology.