c-Jun causes focus formation and anchorage-independent growth in culture but is non-tumorigenic.

The RCAS retroviral vector was used to express chicken and mouse cellular Jun proteins in chicken embryo fibroblasts. Both mouse and chicken proteins induced foci of transformed cells with low to moderate efficiency compared with viral Jun, but were as effective as the viral protein in promoting anchorage-independent growth. Viral Jun and a recombinant between viral and cellular Jun induced tumors in 1-day-old chicks; the cellular Jun proteins were uniformly non-tumorigenic.