Genetic control of expression of endogenous virus genes in chicken cells

Some chicken embryos which are known to be free of virus, nevertheless contain viral products, the group specific (gs) antigen and the virus envelope proteins, which are specific to avian leukosis-sarcoma viruses. The envelope proteins seem to confer these cells with the helper activity for defective sarcoma virus. The formation of these products appears to be determined by a single pair of autosomal genes. The presence of the products is dominant over the absence. Thus, by characterizing the genotype of each individual chicken, it was possible to breed a flock of chickens that are negative for the viral expression. With over 95% of the embryos, two virus-specific properties, the formation of gs antigen and helper activity, were either expressed or unexpressed concurrently. However, some embryos display unusually high levels of helper activity with low or undetectable amounts of gs antigen. In this type of embryo, the formation of particularly high titers of virus was observed only for the virus which is formed by the assistance of the endogenous helper factor. Evidence suggests that this property is also genetically controlled.