A serotype 3 Dearing strain (T3D) of reovirus (Respiratory Enteric Orphan virus) with potential oncolytic activity. Reovirus, a dsRNA virus, is able to replicate specifically in cancer cells bearing an activated Ras pathway. In contrast to normal cells, two-thirds of human cancer cells are Ras-activated. Unlike normal cells, Ras-activated tumor cells are deficient in host cellular protein kinase R (PKR) activity and so are unable to mount an antiviral response. In Ras-activated tumor cells, reovirus freely replicates and induces apoptosis; tumor cell lysis frees progeny viral particles to infect surrounding tumor cells. A cycle of infection, replication and cell death may continue until Ras-activated tumor cells are eradicated. PKR (eukaryotic translation initiation factor 2-alpha kinase 2) is activated by viral synthesis of double-stranded RNA and is an important component of innate immunity in vertebrates, protecting against viral infection.