passages.id: split_1_test_749_passage
passages.type: progene_text
passages.text: These DNA rearrangements , responsible for immune system diversity , are mediated by a site specific recombination machinery via recognition signal sequences ( RSSs ) composed of conserved heptamers and nonamers separated by spacers of 12 or 23 nucleotides ( Lewis , S.M. , 1994. The mechanism of V(D) J joining : lessons from molecular , immunological and comparative analyses. Adv. Immunol. 56 , 27 - 150 ) .