Abstract
Somatic recombination events are responsible for assembling the variable regions of Immunoglobulin and T cell receptor genes from germline-encoded DNA segments (Tonegawa, 1983; Lewis and Gellert, 1989). These rearrangements are mediated by a recombination activity that recognizes signal sequences (consisting of conserved heptamer and nonamer elements separated by nonconserved spacer regions of 12 or 23 nucleotides) located adjacent to the V, D, and J coding segments. Although the mechanism of the reaction remains obscure, recombination is thought to involve either single-stranded or double-stranded cleavage at the border between a signal heptamer and a coding segment, followed by rejoining of the DNA ends in a new configuration (Alt and Baltimore, 1982; Lewis and Gellert, 1989).