The regional genes (V, D, J), used to generate T-cell receptors and Immunoglobulin molecules, are flanked by recombination signal sequences (RSSs) that are recognized by a group of enzymes known collectively as the VDJ recombinase. RSSs are composed of seven conserved nucleotides (a heptamer) that reside next to the gene encoding sequence followed by a spacer (containing either 12 or 23 unconserved nucleotides) followed by a conserved nonamer (9 base pairs). The RSSs are present on the 3’ side (downstream) of a V region and the 5’ side (upstream) of the J region. These are the sides that will be involved in the joining. Only a pair of dissimilar spacer RSSs are efficiently recombined (i.e. one with a spacer of 12 nucleotides will be recombined with one that has a spacer containing 23 nucleotides). This is known as the 12/23 rule of recombination (or the one-turn/two-turn rule).