V(D)J recombination is a DNA recombination process that occurs during the development of immune cells, generating diversity in immunoglobulin (Ig) and T-cell receptor (TCR) proteins (Schatz and Ji 2011). This system derives its name from the coding regions that are assembled to make functional immunoglobulins and T-cell receptors, the variable (V), diversity (D), and joining (J) regions. Proteins dedicated to this process, called RAG proteins, mediate recombination between V(D)J coding sequences, contain DDE-type transposase motifs, and are evolutionarily related to Transib transposases. Similar to the recombinase used by hAT transposons, RAG proteins mediate intramolecular DNA hairpin formation, which causes a double-strand DNA break at each of the boundaries of the V(D)J coding sequences. A nonhomologous end-joining DNA repair pathway is used to fuse different V(D)J sequences together following the action of the RAG proteins.