(Top) Schematic representation of the MTS locus according to Cayuela et al.5 7 The transcripts initiated from the MTS2 promotor and from the two MTS1 promotors (MTS1α and MTS1β) are represented. Their protein products are indicated. The principal probes used for Southern blot analysis of the MTS locus configuration in T-ALL samples are shown. Horizontal arrows indicate the direction of transcription. (Bottom) Representation of 176 alleles with respect to the configuration of the locus. *, the configuration shown corresponds to the three cases in which the two breakpoints have been precisely localized (T39p, T117, and T127). In the last case (T106), the telomeric breakpoint has not been localized.