Detection of 5′ deletions and aberrant Notch1 transcripts in primary murine “thymomas.” (A) Sequences of PCR products obtained by amplification of genomic DNA isolated from 2 thymic lymphomas with primers flanking the most common breakpoints associated with type 1 aberrant transcripts. Sites of DNA breakage and joining, as deduced from sequencing of PCR products, are shown. Residues matching the consensus RAG recognition sequence (CACAGTG followed by a 12- or 23-bp spacer and the sequence ACAAAAAAC) are denoted with an asterisk. N nucleotides and P nucleotides (underlined) are also shown. GL indicates germline DNA flanking the breakpoints. Boxes represent sequences resembling RAG signal sequences. (B) Ratiometric Notch1 quantitative RT-PCR analysis. The relative amounts of transcripts containing 5′ (exons 23 and 24) and 3′ (exons 30 and 31) Notch1 sequences were determined for the tumors in panel A and normal murine thymus, a cell line with a homozygous type 2 deletion (135.2), and a cell line with a heterozygous type 1 deletion (144). Each determination was made in triplicate. The results shown are representative of 2 independent experiments.