Figure 1 | The basic steps of V(D)J recombination.   A | Simplified representation of a germline antigen-receptor locus and recombination signal sequence (RSS). Mammals have seven antigen-receptor loci, each with a different configuration and different number of variable (V), diversity (D) and joining (J) coding segments. B | Stages of the V(D)J-recombination reaction. The recombination-activating gene proteins, RAG1 and RAG2, bind to an RSS and introduce a single-strand nick (a). This RAG–RSS complex synapses with another RSS (b) and uses the hydroxyl (OH) group created by nicking to attack the opposite strand, forming a double-strand break (c). Note that this cleavage reaction leaves two hairpin coding ends and two blunt signal ends, which remain associated with the RAG proteins in a post-cleavage complex (PCC). The post-cleavage complex serves as a scaffold for the DNA ends and might help to recruit factors from the non-homologous end joining (NHEJ) pathway to ligate the ends and form signal and coding joints (d).

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