Proto-oncogene fos is a multifaceted gene, which is expressed during cell growth, cell differentiation, and development. The viral homologue, v-fos, was identified as the resident transforming gene of FBJ-murine osteosarcoma virus which induces bone tumors in mice. Due to an in-fram 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 fas proteins are nuclear in their location, and can transform fibroblasts in vitro. However, transformation by c-fos gene requires removal of a 67 base pair sequence from the 3' non-coding domain. Proto-oncogene fos is a highly inducible gene in response to a variety of growth factors and differentiation-specific inducers. The transcriptional enhancer and the inducible element of the fos gene have been mapped along with sequences in the 3' non-coding domain which may influence the stability of the c-fos transcripts. The c-fos gene transcription is regulated by positively and negatively acting cellular factors.