This system allows the investigators to explore the functional properties of individual viral genes of VHSV in detail. This system was immediately utilized to characterize the non-virion (NV) gene of novirhabdoviruses. Even though it has been demonstrated that the NV gene is not necessary for viral replication, it is highly essential for viral pathogenesis. A new role of NV protein has been discovered and demonstrated that it inhibits apoptosis at the early stage of viral infection. This discovery unlocked the mystery of presence of NV proteins in novirhabdoviruses. Virus subtypes Different isolates (unique strains) of VHSV are typically grouped by genotyping. It is found that genotype groups are divided more geographically than by host species. Earlier studies used different numbering systems, but the following system has come into common usage based on genotype similarity based on sequencing of the N- and G-genes. Types I-III are enzootic to Europe, and Type IV to North America, and Type I and type IV isolates are further subdivided, as follows: Type I-a was the only strain known from VHSV's discovery in 1963 until the late 1988, isolated to fish farms in continental Europe, affecting primarily rainbow trout and occasionally brown trout or pike. In 1988, the first marine strain of VHSV, now designated type IV, was found in normal-appearing salmon returning from the Pacific to rivers of Washington State. This strain and other marine strains were not lethal to rainbow trout. The discovery prompted further studies, and by the mid-1990s, marine VHSV was found in eight species along the northern North America's Pacific coast, and 14 species in and around the Atlantic's North Sea. 1996 saw the first VHSV in Japan, among Japanese flounder farmed in the Seto Inland Sea, and different genotypes have appeared in different areas since then. Type IV was later found off North America's northern Atlantic coast, in Atlantic herring (Clupea harengus) mummichog (Fundulus heteroclitus), stickleback (Gasterosteus aculeatus aculeatus), brown trout (Salmo trutta), and striped bass (Morone saxatilis), as well as dozens of freshwater species in the Great Lakes. VHSV continues to be found in new geographical areas, in new species of fish. This is thought to represent both the spread of the virus into new areas, as with VHSV egg and live fish transfers from North America to Asia, or feeding of raw marine fish to inland farmed trout in Finland, as well as discovery of existing populations, as