Chromosomal breaks at the TCR-αδ locus and translocations in T-cell progenitors lacking ATM. (A) Normal karyotype; FISH probe hybridized with 5′-TCR-α/δ (blue) and 3′-TCR-α/δ (green). Red represents the chromosomal paint probe for mouse chromosome 14. E indicates defined transcriptional enhancer elements. (B) Representative images of breaks between 5′TCR-α/δ (blue) and 3′TCR-α/δ (green) on chromosome 14 (red; top panel). The percentage of DN2/DN3a cells and DN3b-DN4 cells with a TCR-α/δ locus break is shown in a bar graph (bottom panel). Thymocyte maturation arrests at the DN2/DN3a stage in RAG2^−/− mice; thus, DN3b-DN4 phase locus breaks cannot be measured. (C) Representative images of a chromosomal translocations (top panel). The percentage of cells with a chromosome 14 translocations is shown in a bar graph (bottom panel). y-axis indicates change to percentage chromosome 14 breaks and translocations. (D) Representative images of chromosomal abnormalities showing the TCR-α/δ locus on a dicentric chromosome 14 (yellow arrow) and breaks (white arrow). (E) Representative image of a chromosomal 12;14 translocation (white arrow) and a dicentric chromosome 14 (yellow arrow). Data are mean ± SE. **P < .01.