The RAD51/RAD52-dependent DNA repair pathway is involved in DNA recombination and DNA double-strand break repair in yeast. Although many proteins in the RAD51/RAD52-dependent DNA repair pathway have been identified in yeast, a novel protein(s) that functions with RAD51/RAD52 may also exist in humans. Using a yeast two-hybrid system, we have identified a 12-kDa protein that associates with the human RAD51 and RAD52 proteins. This protein shares significant amino acid homology with the yeast protein SMT3, which functionally associates with the yeast mitosis fidelity protein MIF2. It also shares moderate homology with ubiquitin and several other proteins, including the N-terminus of the RAD23 protein and a ubiquitin cross reacting protein. Therefore, the gene is tentatively designated UBL1 for ubiquitin-like 1. The UBL1 mRNA is expressed in many human tissues, most highly in testis. The UBL1 gene is mapped to chromosome 2q32.2-q33, and a related sequence may be located on chromosome 1q23-q25.