Mechanism for insertion of the

BCL2 gene into the IGHlocus. (A) Cryptic RSSs at both sides of the BCL2 gene form a synaptic complex followed by excision of the BCL2gene. The hairpin ends of the remaining 18q21 locus are opened, processed, and ligated. (B) At the 14q32/IGH locus, excision of DH and JH sequences takes place, but without ligation of the coding ends. The coding end hairpins are opened and processed, and they finally form hybrid joints with the excised BCL2 gene. The hairpins are indicated with small circles; N-nucleotide insertions are indicated with thick lines and “NNN”; and RSSs are symbolized by CAC for forward or GTG for reverse.