Amplification of T(12;15) translocation breakpoint regions by segmental jumping translocation (SJT) in BALB/c mouse plasmacytoma, MOPC 315. (A) SKY display image of MOPC 315. Rearrangements between Chrs 12 and 15 (indicated by arrows) were observed consistently in 30 images on both the plasmacytoma-specific chromosome T(12;15) and the marker chromosomes, T(12;15;16;12;15;16) and T(17;15;16;12;15;16). The chromosomes are numbered I, II, and III, respectively. (B) Reverse painting of flow-sorted tumor chromosomes on normal mouse chromosomes stained with DAPI. The normal DAPI-stained chromosomes are shown in the white insets to facilitate the interpretation of the reverse painting results. Three distinct flow peaks designated I, II, and III were identified (not shown). They must have contained the translocated chromosomes I, II, and III (shown in [A]) for the following reasons. Peak “I” contained Chr T(12;15) because the translocation juxtaposed the Ig heavy-chain gene cluster, located on Chr 12F2, to the Myc locus, residing on Chr 15D2. Therefore, upon reverse painting, Chr 12 should be labeled completely in red (with the exception of the small telomeric cap distal to band F2 that is not discernible in the image due to its small size), whereas Chr 15 should be labeled from band D2 to the telomere. The observed reverse-painting pattern depicted at the top matched this expectation. Peak “II” contained Chr T(12;15;16;12;15;16), because the FISH probe derived from it stained the distal half of Chr 16, but not Chr 17. Peak “III” contained Chr T(17;15;16;12;15;16), because the FISH probe obtained from it stained both Chr 16 and Chr 17. Thus, the reverse painting pattern of the three flow peaks corresponded to the structure of the translocated chromosomes as predicted by SKY. (C) Detection of the same clonotypic junction fragment between the switch region (S) of the Ig heavy chain  gene (C) and intron 1 ofMyc by direct, two-round PCR amplification with nested primer pairs (arrowheads). The identical hybrid fragment (indicated by the two-colored horizontal bar) was obtained when DNA samples prepared from the flow-sorted marker chromosomes I, II, and III were used as templates in three different PCR reactions. DNA sequence analysis confirmed the identity of the Myc/S breaksite and its flanking regions on all three chromosomes. Twenty basepairs ofMyc and S are shown at the bottom to left and right of the breakpoint, respectively. Exons 2 and 3 of Myc, exon 1 of C, and S are depicted as labeled boxes. The T(12;15) translocation breaksite is indicated by an arrow.