XIRG (for Xenopus IRG) was cloned by screening a cDNA library of UV-ventralized stage 13 Xenopus laevis embryos for specifically ventrally expressed mRNAs. Embryonic XIRG mRNA expression is restricted to non-neural ectoderm at the gastrula and neurula stages. In adult X. laevis, XIRG mRNA can be detected in skin and kidney. Extensive searches in nucleic acid and protein databases revealed homologous sequences in mouse, human and zebrafish. Mouse IRG1 mRNA is expressed in cultured macrophages as a response to bacterial lipopolysaccharide treatment.