JUN and osteosarcoma: While c-Jun, c-Fos and FosB efficiently transform mammalian cells in vitro (Jochum et al., 2001), only c-Fos overexpression causes osteosarcoma formation (Grigoriadis et al., 1993), whereas c-Jun is required for development of chemically induced skin and liver tumors in mice (Young et al., 1999; Eferl et al., 2003).