Examples of class-switch rearrangements on the translocated allele of FLs, detected with a combination of IgH and BCL-2 probes. The red signal on the left side of each fiber is a BCL-2 cosmid probe located about 20 kb 5′ from the major breakpoint cluster (MBR) and 50 kb 5′ from the minor cluster region (MCR) of BCL-2. The location of the translocation breakpoint (derived from hybridizations with multiple BCL-2 probes, data not shown) is indicated with a white line. Originally, a more proximal (3′) BCL-2 cosmid was used that covers the breakpoint in case of a major breakpoint translocation. The more 5′ cosmid in this figure was used to create a gap between BCL-2 and IgH signals and hence facilitate interpretation of the IgH configuration. The upper 2 fibers show translocations with an MBR-breakpoint and with class switching to Cγ4 and Cγ2. The lower 3 fibers have translocation breakpoints in the MCR. Two of these have class switching to C1 and Cγ1, respectively. The bottom one has retained the Cμ-Cδ region but has downstream rearrangements, apparently consisting of deletion of a single Cγ gene from both the 5′ and the 3′ C_Hcluster.