We cloned and sequenced the cDNAs which code for rat cellular nucleic acid binding protein (CNBP). In-frame insertion/deletion differences were found among the clones at two sites in the open reading frame, suggesting alternative splicing of the message or the presence of multiple genes which code for this protein. The deduced amino acid sequence revealed that one rat CNBP sequence was completely identical to its human counterpart. This striking conservation, together with the fact that homologous genes have been found in various organisms including Schizosaccharomyces pombe, suggests that CNBP plays a basic biological role in eukaryotic cells. The recombinant GST-CNBP fusion protein produced in Escherichia coli bound to a G-rich single-stranded RNA and DNA in a sequence-specific manner.