Proto-oncogene fos, which is expressed during cell growth and cell differentiation and development, is a multifaceted gene. The viral homolog, v-fos, was identified as the resident transforming gene of FBJ-murine osteosarcoma virus, which induces bone tumors in mice. Owing to an in-frame deletion during the biogenesis of the v-fos gene, the products of viral and cellular fos proteins differ at their C-termini. Despite different C-termini, both fos proteins are nuclear in their location and can transform fibroblasts in vitro. However, transformation by the c-fos gene requires removal of a 67-base pair sequence from the 3' noncoding domain. Proto-oncogene fos is a highly inducible gene in response to a variety of growth factors and differentiation-specific inducers. The expression of the fos gene is not modulated during the cell cycle.