Atm^−/− thymocytes express Tcrb transcripts of unrestricted diversity. (A) Normal levels of TCRβic in ATM-deficient CD4⁺CD8⁺ DP thymocytes. (B) Diverse CDR3 length of selectable (Vb1, Vb8) or nonselectable (Vb17) Tcrb transcripts in individual Atm^−/−, Atm^+/−, and Atm^+/+ thymi. For Vb1 the arrow indicates a dominant clonal rearrangement in one Atm^−/− animal, probably a preleukemic clonal expansion. (C) Schematic representation of the Vb^a and Vb^b haplotypes (the former contains a deletion of approximately half of the Vb gene segments and a functional Vb17 gene, whereas the latter contains the Vb17 pseudogene, ψ17).⁴⁰  For Vb17, the arrow indicates a rearrangement that falls out of the normal CDR3 length distribution.