Recombination-activating gene 1 (RAG1), as well as RAG2, are the only lymphoid-specific genes required for V(D)J recombination. RAG1 protein contains a C3HC4 zinc-binding motif (zinc ring finger) that binds two zinc ions. We have found that RAG1 contains additional zinc-binding motifs in the form of two separate C2H2 zinc finger sequences. One of the zinc fingers, in combination with the C3HC4 subdomain, forms a highly specific dimerization domain. A combination of biophysical techniques has been used to determine the energetics of association, the overall shape of the dimerization domain, and the relative orientation of the monomeric subunits within the dimer. These results provide direct evidence that a C3HC4 motif is involved in a protein-protein interaction, in this case via homodimer formation. In addition, the observation that the dimerization domain includes multi-class zinc binding motifs, namely both a zinc finger and a C3HC4 subdomain, has important implications for other C3HC4-containing proteins. The position of this dimerization domain in the N-terminal third of the RAG1 sequence of 1040 amino acid residues may have a significant influence on the activities associated with the C-terminal domains of the protein.