v(D)J recombination: for the community of immunologists and developmental biologists, the molecular route by which B and T lymphocytes acquire their unique function of affording adaptive immunity. Yet, for many-from experienced scientists to trainees-it represents a (rather too) sophisticated process whose true insight is excessively demanding. However, when not simplyconsidered as a private ground for a few aficionados, it can be seen as a way of understanding how maturelympho cytes carry on their basic functions. For the group of aficionados-which includes this editor-it is an elegant paradigm featuring many fascinating evolutionary achievements of which the biological world alone has the secret. These include a subtle biochemical principle most likelyhijacked some 470 million years ago from an ancestral gene invader and since then cleverly adapted by jawed vertebrates to precisely cleave and rearrange their antigen receptor (Ig andTCR)loci. This invader would itself have assigned the services of the nonhomologous end joining (NHEJ) DNArepair machinery as well as various DNApolymerases or transferases to work in concert with developmental clues in lymphoid cell lineages to generate an immune repertoire and efficient host surveillance while avoiding autoimmunity. Recently, important new refinements in these systems have emerged, continuing to challenge ourknowledge andbeliefs. These arejust thetopics covered by the senior authors-all established leaders in this field-and their colleagues, whilst writing the various chapters in V(D)J Recombination.
