Analysis of TCRα expression and Ja50 recombination in ATM-deficient thymocytes. (A) Western blot analysis using anti-TCRα performed on thymocyte postnuclear extracts from mice segregating the Atm mutation, wild-type C57BL/6, and Tcra^−/− controls. Reprobing with anti–β-actin verified equal loading. The numbers below each lane represent densitometric quantification of the relative amount of TCRα compared with the loading control. (B) Total thymocyte postnuclear extracts from the indicated mouse strains were immunoprecipitated with anti-TCRβ (H57-597) or hamster IgG isotype control antibodies and resolved by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions. Western blot analysis was performed with anti-TCRα. (C) Quantitative reverse transcriptase (RT)–PCR of TCRαC transcripts in thymocytes from Atm^+/+ (▪) and Atm^−/− (□) mice. The graph depicts the mean ± SD measurements for 2 individual Atm^+/+ and 3 individual Atm^−/− mice. (D) Ja50 rearrangements but not unrepaired CEs can be detected in Atm^−/− thymocytes. In one 12-week-old Atm^−/− animal, we observed expansion of a clone that had already undergone Tcra rearrangement (last lane). A restriction map of the 5′ end of the Tcra locus indicates the expected sizes of the products generated by digestion at the relevant restriction enzyme sites.