Oncogenic Nras-initiated TALL tumors contain clonal Notch1 mutations. (A) Southern blot analysis of genomic DNA obtained from TALL-het and TALL-homo tumors. The blot was hybridized with a probe to the Vβ region of T-cell receptor β. (B) Sequence analysis of the exon 34 of Notch1 in control, TALL-het, and TALL-homo thymocytes.