The Rel family of transcriptional activators form a large diverse group of proteins that are involved in the activation of genes involved in immunity, development, apoptosis and cancer. So far, none of the rel genes cloned in mammals appear to be required for embryonic development. We have cloned and characterized a cDNA from an embryonic cDNA library that encodes a novel Xenopus Rel protein, called Xrel3. Xrel3 is a member of the cRel subfamily and is most closely related to but distinct from other Xenopus Rel members. The expression of Xrel3 mRNA was investigated using Northern analysis, RNase protection assay, reverse transcriptaselinked polymerase chain reaction and in situ hybridization. Messages are present maternally and are slightly enriched in the equatorial region of the blastula stage embryo. At gastrulation, the accumulation of Xrel3 messages declines to undetectable levels but then increases after neurulation. In situ RNA hybridization was used to determine the spatial location of Xrel3 messenger RNA in embryos. Messages are localized to the developing forebrain, dorsal mid-hindbrain region, the inner ear primordium, or otocyst, and in the notochord. Overexpression by microinjection of Xrel3 RNA induced tumors in the developing embryo that appeared after gastrulation. The location of the tumors depended on the location of the injection site. These results suggest that Xrel3 might have a generalized role in regulation of cell differentiation in the embryo.The Rel family of transcriptional activators is a group of higher eukaryotic proteins that participate in cell differentiation (for reviews, see Refs. 1-7). The family includes the c-Rel oncoprotein, RelB, NF-B (both p65 or RelA, and p50 subunits), and the DORSAL patterning morphogen of Drosophila. They are expressed in a wide variety of tissues and transactivate genes that are involved in the immune response, apoptosis, cancer, and development. Rel transcription complexes exist as homo-or heterodimers, but only specific dimer pairs have been demonstrated in vivo.Although there are different levels of rel gene regulation in different cells, the activity of all of the proteins is determined by their subcellular localization. Thus, Rel proteins are active in the nucleus, but are retained in the cytoplasm in an inactive state. Depending on the cell type, Rel complexes activate transcription from target genes in either a constitutive or inducible manner. Inducible complexes are retained in the cytoplasm by a regulatory subunit (1-3). Upon induction, the regulatory subunit releases the Rel complex and allows it to translocate to the nucleus where it binds to target DNA sequences.Rel proteins share motifs required for their similar functions. The amino terminus contains the highly conserved Rel Homology Domain (RHD) 1 of about 300 amino acids, which is required for DNA binding, dimerization with other Rel proteins, and a nuclear localization signal. The remainder of the protein is variable and is necessary for transcriptional activation (8). In some cases...