Herpesvirus dna can be distinguished on the basis of size, base composition and structural arrangement in unique and repeated sequences. the length of herpesvirus dna varies from approximately 120 to more than 230 kilobases (kb) and is characteristic for each genus (e. g. simplex virus and cytomegalovirus). minor variations in the size of individual isolates of the same virus are due mainly to variations in the number of internal and teninal repeat regions. herpesviruses can be divided into six structually distinct groups on the basis of the presence and location of repeated sequences greater than 100 base pairs. in group a viruses, exemplified by the human herpesviruses 6 and 7 (hhv6 and hhv7), a large sequence from one terminus is directly repeated at the other terminus (left terminal repeat and right terminal repeat). in group b viruses, exemplified by kaposi's sarcoma herpesvirus (hhv8; see the monograph in this volume), the primate herpesviruses saimiri (hvs or shv-2) and ateles (hva-2) and the mouse herpesvirus strain 68 (mhv-4), the terminal sequence is directly repeated numerous times at both termini. in group c viruses, exemplified by epstein-barr virus (ebv, hhv4; see the monograph in this volume), both terminal and internal repeat sequences are present throughout the viral genome, which subdivide it into well-defined unique sequences. in group d viruses, exemplified by varicella–zoster virus (vzv, hhv3) and by numerous viruses isolated from mammals and birds, the terminal region is repeated in an inverted orientation internally. the short unique region contained between the terminal repeat and this inverted form can assume two possible orientations relative to the long unique sequence, such that virions isolated from infected cells consist of two equimolar populations. a more complex architecture is observed in group e viruses, exemplified by herpes simplex viruses types 1 (hsv-1, hhv1) and 2 (hsv-2, hhv2), human cytomegalovirus (cmv, hhv5) and marek's disease virus (mdv or ghv-2). sequences from both termini (anb and anc) are repeated in inverse orientation and juxtaposed internally (a′nb′ and a′nc′), dividing the genome into two components consisting of long and short unique sequences separated by inverted repeats. both components can occur in two orientations relative to each other, resulting in four equimolar populations of virions. there are no repeat regions in the genome of the group f viruses, exemplified by murine herpesvirus 1.