IBDV belongs to the genus Avibirnavirus of the family Birnaviridae that are nonenveloped, icosahedral double-stranded RNA viruses enclosing two segments, A and B. The short segment B (2·8 kb) comprises viral protein 1 (VP1, 'an RNA-dependent RNA polymerase'; 97 kDa), while segment A (3·17 kb) encodes the major components of the virus and contains two partially overlapping open reading frames (ORFs). The first ORF encodes the nonstructural viral protein VP5 (17 kDa) and the second ORF encodes a polyprotein precursor (pVP2-VP4-VP3, 110 kDa) that can be trans-cleaved by VP4 (28 kDa) to release pVP2 (512 residues, 54·4 kDa) and VP3 (32 kDa). The puromycin-sensitive aminopeptidase (PurSA) and VP4 cleave pVP2 at its C-terminus to produce the transitional pVP2 (452 residues). pVP2 is further cleaved to produce the mature VP2 (441 residues). VP2 (206 to 350 aa residues) is encoded by the hypervariable region (HVR) of IBDV and is responsible for antigenic variation. In addition to homologous recombination in segments, mutations in the HVR of VP2 and genetic reassortment events are also responsible for the variation in IBDV.