We have previously described the distribution of a surface glycoprotein recognized by monoclonal antibody B721. We now report the molecular characterization of this molecule at the protein, cDNA, and genomic level. A 75-kDa glycoprotein can be immunoprecipitated from B721+ tissues. We have isolated a near full-length cDNA containing the complete coding region and a full-length genomic clone. We present evidence that this protein has similarity to several classes of nuclear transcription factors, particularly the myc family of proteins. The 721P protein was found to have a leucine zipper-like structure, a possible basic region immediately upstream from the leucine zipper, and a protein kinase A phosphorylation site. 721P protein is encoded by a gene distinct from any deposited in existing data bases, and displays several features associated with proteins involved in signal transduction and gene regulation.