The program MLOGD models and compares sequence evolution in single-coding and dual-coding sequences. It has previously been used to identify a second ORF, in a different but overlapping reading frame from that encoding the viral helicase (VP6 of BTV), within Seg-9 of the insect-borne orbiviruses [17, 18, 30] . This ORF was also identified in tick-borne orbiviruses [18] . The length of the putative translation product is highly variable, even between closely related Orbivirus species. In BTV and EHDV it is approximately 10 kDa, in Peruvian horsesickness virus (PHSV) and Yunnan orbivirus (YUOV) it is approximately 13.5 kDa, while in AHSV it is approximately 17 kDa, and in GIV it is approximately 22.5 kDa (twice as long as in BTV). These NS4 sequences contain a high proportion of charged residues, with basic R+K (arginine + lysine) content ranging from 13% to 22%, while acidic E+D (glutamic + aspartic acids) content ranges from 12% to 22%. Each NS4 protein contains 4-5 histidine residues, with the exception of the BTV protein, which contains none.