Restriction map of the MTS locus and schematic representation of its configuration in T-ALL samples. (A) An updated MTS locus map is shown. The probes used and the 2 breakpoint clusters MTS1^bcrα and MTS1^bcrβ are indicated. C5.3, 2.7, and 2.3, sequence tagged sites (STSs); E, exon; *, polymorphic BamHI site; +, the 16 breakpoints include that of the T84 sample, for which the MTS1 configuration was not fully characterized. (B) Schematic representation of the 2 chromosome 9s in cases with rearrangements occurring in the locus (22 T-ALL samples and the MOLT4 cell line).21 In 1 sample, T106, 2 breakpoints occurring on each chromosome 9 were detected. In T39, breakpoints were different at presentation (T39p) and relapse (T39r). Dashed lines indicate deletions; vertical arrows indicate breakpoint-containing regions. The T84 configuration is not shown. All rearrangements within MTS1^bcrβ suppress the possibility to express p16^INK4a (encoded by exon 1α, exon 2, and exon 3) and p19^ARF (encoded by exon 1β, exon 2, and exon 3). In rearrangements within MTS1^bcrα, MTS1 exon 1β is deleted and p16^INK4a encoding exons remain unchanged. Transcripts initiated from the promoter located 5′ to MTS1 exon 1α and the p16^INK4a protein are expressed (data not shown). These data suggest that p19^ARF and/or p15^INK4b and not p16^INK4a could be the functional target(s) of the rearrangements in these cases (Gordie et al, submitted for publication).