c-Fos belongs to a family of transcription factors, including FosB, Fra1, Fra2. The fos oncogene was isolated as a retroviral transforming gene (v-fos) carried by the FBJ-murine osteosarcoma virus. c-Fos undergoes post-translational modifications and other forms have been described including 57 Da, 60 kDa and 62 kDa proteins. Stable expression of c-Fos has been shown in developing bone tissue and teeth, hematopoietic cells,germ cells, and in the central nervous system. High levels of c-Fos are expressed in human full term fetal membranes.