The RAG1 and RAG2 gene products are indispensable for activating somatic rearrangement of antigen receptor gene segments. The two proteins form a stable complex in primary thymocytes as well as when expressed in adherent cells. In both cell types, most cells localize RAG proteins at the periphery of the nucleus. However, when overexpressed in fibroblast cells, RAG1 is found largely in the nucleolus. Nucleolar localization of RAG1 is mediated by several domains containing stretches of basic amino acids, indicating that RAG1 has affinity for RNA or ssDNA. The RAG1 interacting proteins SRP1 and Rch1 directly bind to the nuclear localization signals of RAG1, which mediate the nuclear and nucleolar translocation of the protein. RAG1 appears to have a binary structure, each half containing multiple regions that can act as NLSs, binding sites for the SRP1/Rch1 family, and RNA binding domains.