We have screened a human adult iris cDNA library to identify genes that are highly expressed and conserved between humans and pigs. We identified human iris cDNAs that hybridized at high stringency to a porcine choroidal ring cDNA probe. Of 1568 human iris cDNAs examined, 176 were found to have high expression in porcine choroidal rings. One of the 176 clones was identified as a previously uncharacterized cDNA that we have named the Ubiquitin-like 5 gene (UBL5). The UBL5 gene is located on chromosome 19p13.2, and its genomic structure has been examined. There is a UBL5 pseudogene on chromosome 17p11.2. We have also found homologues to the UBL5 gene in Arabidopsis thaliana, Caenorhabditis elegans, Schizosaccharomyces pombe, and Saccharomyces cerevisiae. Northern blot analysis of the Ubiquitin-like gene 5 revealed expression in every tissue tested, with the highest levels of RNA expression in heart, skeletal muscle, kidney, liver, iris, and lymphoblasts. Intracellular localization experiments in COS-7 cells showed that the recombinant UBL5 protein is cytoplasmic. Western analysis demonstrated that the recombinant UBL5 protein is approximately 9 kDa, as predicted from the cDNA. A comparison between UBL5 and its homologues with other Ubiquitin-like proteins and Ubiquitin, using the PROTDIST program, suggests that the UBL5 genes are a separate class of Ubiquitin-like genes. Further characterization of the UBL5 gene will determine the function of the encoded protein and whether it is a candidate for ocular disease.