In most vertebrate species analyzed so far, the diversity of soluble or membrane-bound antigen-receptors expressed by B and T lymphocytes is generated by V(D)J recombination. During this process, the coding regions for the variable domains of antigen-receptors are created by the joining of subexons that are randomly selected from arrays of tandemly repeated V, D (sometimes) and J gene segments. This involves the site-specific cleavage of chromosomal DNA by the lymphocyte-specific recombination-activating gene (RAG)-1/2 proteins, which appear to have originated from an ancient transposable element. The DNA double-strand breaks created by RAG proteins are subsequently processed and rejoined by components of the nonhomologous DNA end-joining pathway, which is conserved in all eukaryotic organisms — from unicellular yeast up to highly complex mammalian species.