In two previous studies we described the properties of a heat-stable DNA-binding protein present in rat liver nuclei. This protein, hereafter termed C/EBP, is capable of selective binding to the CCAAT homology of several viral promoters (Graves et al. 1986), as well as the core homology common to many viral enhancers (Johnson et al. 1987). We now report the isolation of a recombinant clone of the gene that encodes C/EBP. Expression of the clone in bacterial cells yields a protein that binds in vitro to both the CCAAT homology and the enhancer core homology, providing conclusive evidence that a single gene product accounts for both binding activities. By examining the properties of protease-derived fragments of C/EBP, we have localized its DNA-binding domain to a 14-kD fragment. A 60-amino-acid segment located within the DNA-binding domain of C/EBP bears sequence similarity to the products of the myc and fos oncogenes.