Structure of type 1 and type 2 Notch1 transcripts. (A) Products amplified from 144 cells, a line expressing type 1 Notch1 transcripts, using a 3′ exon 27 primer and 5′ linker primer, are shown after agarose gel electrophoresis. Sequences of 5′RACE products are shown below; the basepair numbering is in relationship to the A residue in the ATG start site in exon 1 of the Notch1 gene, which is designated position 1. (B) Northern blot analysis of lines expressing type 1 Notch1 transcripts with probes from the 3′ end of exon 25 and the 5′ end of exon 25. The diagram shows the regions of the Notch1 that are encoded by exons 25 and 27, respectively. (C) Sequences of RT-PCR products obtained from murine T-ALL cell lines expressing type II transcripts. Sequencing of products obtained from 130.1 and 130.2 cells revealed a transcript in which exon 1 is spliced out of frame to exon 27. In SCID-adh cells, a product was obtained in which exon 1 is spliced to an intron 1 sequence that contains an in-frame stop codon, which is in turned joined to an internal sequence within exon 28.