Notch1 mutations in preleukemic thymocytes. (A) Tcrb gene rearrangements in thymocytes from mouse 7151/4 and tumor resulting after transplantation into nude mouse (N5/2). Several oligoclonal rearrangements are evident in the thymocytes, whereas only 2 rearranged bands are seen in the tumor sample. Germ line bands are indicated with arrows. (B) An AluI site in the PEST domain was disrupted by the Notch1 mutation in the N5/2 sample. In 7151/4, the majority of the PCR product was wild type (211 bp), but a faint band (241 bp) indicated a small amount of the mutated allele. Sample 3917 is an SCL/LMO1 tumor that is wild type for the PEST domain used as a control. Restriction map for PEST domain PCR product. A indicates AluI sites; A^*, AluI site disrupted by Notch1 mutation. Size of fragments (bp) is indicated below the map.