Chromosome conformation capture experiments suggest the production of unique chromosomal loops in peripheral B cells anchored by Eμ and 3′RR physical interactions 19, 20. In the case of CSR, recruitment check details and transcription of the switch-acceptor region in close proximity to the switch μ donor region (switch–switch synapsis) might be promoted by the 3′RR itself 19. Combined mutations of both the Eμ and 3′RR would be the most appropriate tool to address this issue. Ongoing recombination and mutation during B-cell development make the IgH locus a hotspot for translocation. Many lymphomas are marked by proto-oncogene translocation into the IgH locus (Fig. 2A). Bcl-2 and cyclin

D1 translocations, found respectively in follicular and mantle cell lymphoma, take place during the V(D)J recombination 21, 22. The cyclin D1, cyclin D3 or c-maf translocations often observed in myelomas are obviously linked to CSR 23. However c-myc translocation, a typical hallmark of Burkitt lymphoma, PLX3397 is related to either SHM or CSR 24. Among IgH cis-regulatory elements, Eμ was expected to be the critical c-myc deregulator in lymphomagenesis. However, Eμ-c-myc transgenic mice expressed c-myc in B-cell

progenitors and, thus, developed an immature form of lymphoma 25 that differed from human Burkitt lymphoma tumors that harbor a mature B-cell signature 24. In Burkitt lymphoma, c-myc translocation

breakpoints occur either in the V(D)J (endemic Burkitt lymphoma) or in switch regions (sporadic Burkitt lymphoma), both of which keep the 3′RR intact. Thus, transgenic models confirm that the 3′RR is a good candidate for oncogene deregulation (Fig. 2B). c-myc-3′RR transgenics developed Burkitt lymphoma-like Pyruvate dehydrogenase proliferation within a few months 26 and similarly, the knock-in of a 3′RR cassette upstream of the endogenous c-myc gene induced B-cell lymphomas 27. Interestingly, the phenotype of lymphoproliferation induced by the c-myc-3′RR transgene varied with the genetic background. C57Bl/6 animals developed Burkitt-like lymphoma 26, while no lymphoproliferation occurred when the c-myc-3′RR transgene was bred in a Balb/c background (known to harbor a polymorphism of p16Ink4a) 28. p16Ink4a is an inhibitor of cyclin-dependent kinase (Cdk)-4, a regulatory protein implicated in the first steps of cell cycle entry. Breeding c-myc-3′RR C57Bl/6 animals in a Cdk4R24C mutant background (a dominant Cdk4 oncogene resistant to inhibition by p16Ink4a and other members of the Ink family 29) resulted in susceptibility to mantle cell lymphoma (Vincent-Fabert et al., submitted). A convincing demonstration of the essential contribution of the 3′RR in lymphomagenesis has been produced by transgenic models of B-cell lymphomas with IgH-c-myc translocations 30.