V(D)J recombination is instigated by the recombination-activating proteins RAG1 and RAG2, which catalyze site-specific DNA cleavage at the border of the recombination signal sequence (RSS). Although both proteins are required for activity, core RAG1 (the catalytically active region containing residues 384-1008 of 1040) alone displays binding specificity for the conserved heptamer and nonamer sequences of the RSS. The nonamer-binding region lies near the N terminus of core RAG1, whereas the heptamer-binding region has not been identified. Here, potential domains within core RAG1 were identified using limited proteolysis studies. An iterative procedure of DNA cloning, protein expression, and characterization revealed the presence of two topologically independent domains within core RAG1, referred to as the central domain (residues 528-760) and the C-terminal domain (residues 761-980). The domains do not include the nonamer-binding region but rather largely span the remaining relatively uncharacterized region of core RAG1. Characterization of macromolecular interactions revealed that the central domain bound to the RSS with specificity for the heptamer and contained the predominant binding site for RAG2. The C-terminal domain bound DNA cooperatively but did not show specificity for either conserved RSS element. This domain was also found to self-associate, implicating it as a dimerization domain within RAG1.