The EWS gene was found at the chromosome breakpoints in Ewing sarcoma, and the FUS/TLS gene was found at the breakpoints of myxoid liposarcoma and acute myeloid leukemia. These genes encode proteins that carry a highly homologous RNA binding domain. Fusion proteins made of the N-terminal half of EWS or FUS/TLS and transcriptional regulatory proteins, also derived from genes located at breakpoints, have been suggested to be involved in the pathogenesis of tumors. By PCR amplification of human Namalwa cell cDNA using degenerate primers made from the conserved amino acid sequences in the RNA binding domain of EWS and FUS/TLS, we obtained a cDNA fragment (RBP56 cDNA), the predicted amino acid sequences of which were similar but not identical to those of EWS and FUS/TLS. Using this fragment as a probe, we obtained two isoforms of cDNAs consisting of 2144 and 2153 bp, respectively, which encode proteins consisting of 589 and 592 amino acid residues, respectively. The predicted amino acid sequences of RBP56 protein have a serine-, tyrosine-, glutamine-, and glycine-rich region in the N-terminal region, an RNA binding domain and a C2C2 finger motif in the central region, and degenerate repeats of DR(S)GG(G)-YGG sequences in the C-terminal region. The expression of RBP56 mRNA was observed in all of the human fetal and adult tissues examined, as was the expression of EWS and FUS/TLS mRNAs. The RBP56 gene was mapped to chromosome 17q11.2 to q12.