Abstract
Formation of a specific type of infectious Rous sarcoma virus called RSV(0) has been found to occur only in a certain type of chick embryo cell in the absence of avian leukosis virus. Although these chick cells lack any complete form of avian leukovirus, they appear to carry a genetic factor essential for formation of infectious RSV(0). A factor-deficient cell can be converted to a producer of infectious RSV(0) by infection with Rous sarcoma virus or avian leukosis virus which has been grown in factor-containing cells. Evidence is presented to show that the factor determines both the antigenic and host range specificity of RSV(0).