A. JH3-3’RR interactions were analyzed in thymocytes, 3-1 pre-B cells, and splenic B cells. Hole, the nearest non-Ig gene downstream of the 3’ RR, was used as a negative control. B. A schematic description of DJ/VDJ rearrangements on expressed (C57BL/6) and non-expressed (DBA) IgH alleles in 70Z/3 pre-B cell line. The J1 and hs4 primers were paired to detect 3C interaction only on the expressed allele. J3 and hs3a/hs1,2 were paired to detect 3C interactions only on the non-expressed allele. 3C interactions were measured in T cells from DBA mice (negative control), 70Z/3 pre-B cells, and DBA splenic B cells (positive control). Specific interactions were observed on the expressed allele of 70Z/3 pre-B cells.