My lab demonstrated that one Igk allele directs one Igh allele to a common pericentromeric cluster; where thetwo transiently associate; and that pairing of the two loci in this repressive nuclear compartment at the pro- topre-B cell transition induces decontraction of the Igh locus; which is important for establishing allelic exclusionand preventing ongoing cleavage. Intriguingly; we have now discovered that Ig? mRNA is involved in thisregulation. In this application we aim to identify the salient features by which Ig? mRNA alters accessibility tocontrol two important aspects of recombination: allelic exclusion and coordinate rearrangement of the Igh andIgk loci.