In 1958, Harry Rubin and Howard Temin developed an assay where chicken embryo fibroblasts could be altered morphologically by RSV infection. Two years later Temin concluded that the transformed morphology of the cells was controlled by a genetic property of RSV. At that time it was unknown, but later the "src" gene was identified as responsible for morphological transformation in healthy cells. During the 1960s, two findings emerged: replication-competent isolated viruses were related to RSV, but were non-transforming, and an isolated replication-defective strain of RSV was transformation-competent. These two findings gave rise to the notion that viral replication and malignant transformation are separate processes in RSV.