one hundred and fifty copies of the σ2 protein that 'clamp' adjacent λ1 monomers together. At the 12 five-fold axes of symmetry, pentamers of the λ2 protein form turret-like structures that protrudes from the surface of the shell. In the center of the λ2 turret a channel allows viral mRNAs to be extruded during transcription. The channel is 70Å at its base and 15Å at its narrowest point. The core also contains within it twelve copies of λ3, the RNA-dependent RNA polymerase. One λ3 protein is found slightly offset from each of the twelve pentameric λ2 turrets. Closely associated with λ3 are one or two copies of μ2, a transcriptase cofactor. μ2 has been found to have some enzymatic functions, such as NTPase activity. The λ3 protein is responsible for transcription of the double-stranded RNA genome segments. Each transcript is threaded through the λ2 pentameric turret as it is being extruded. Guanylyltransferase enzymatic activity in the λ2 turret adds a 5' guanosine cap to the extruded mRNA. In addition, two methyltransferase domains found in the λ2 structure act to methylate the 7N position of the added guanosine and the 2' O of the first templated nucleotide, which in all cases is also a guanosine. The outer capsid (T=13) is composed of μ1 and σ3 proteins with λ2, in compound with σ1, interspersed around the capsid. It has been proposed that λ2 is involved in replication due to its placement at the fivefold axes and its ability to interact with λ3 in solution. σ1, a filamentous trimer extruding from the outer capsid, is responsible for cell attachment by interacting with sialic acid and other entry receptors. μ1 and σ3 are both involved in the attachment and thus entry of the virus via receptor-mediated endocytosis involving the formation of clathrin-coated pits. Strains Mammalian orthoreoviruses The only orthoreovirus to not produce syncytia, mammalian orthoreoviruses have the capability of infecting all mammals, but do not cause disease, except in young populations enabling them to be studied frequently as a model for viral replication and pathogenesis. Pteropine orthoreovirus This orthoreovirus has been extracted from the heart blood of a fruit bat (Pteropus policephalus) in Australia with different viruses being isolated from different species, such as the flying fox (Pteropus hypomelanus) which have been found to cause respiratory infections in humans in Southeast Asia. The Nelson Bay Orthoreovirus, like the Avian orthoreovirus, has 3 open reading